By default, Mac starts from its built-in hard disk, but a startup disk can be any storage device that contains bootable contents that compatible with your Mac. For example, if you install macOS or Microsoft Windows on a USB drive, your Mac can recognize that drive as a startup disk. This guide provides 2 ways to boot a Mac from a USB flash drive.

Requirement

Starting up your Mac from an external disk requires the following:

• Intel-based Mac.
• Bootable USB thumb drive formatted with a GUID partition type and containing an OS X installer or a usable operating system.

Let’s see how to boot a Mac from a bootable USB drive and what to do if your Mac doesn’t start up from it.

Way 1: Boot Mac from USB Drive using Startup Manager

Macbook Pro refuses to mount an specific external hard drive or USB stick, after being able to do so with no problem before, and usually after the operating system disconnects it suddenly. That particular external hard drive or USB stick is perfectly good since it works OK under linux. MBP would mount the drive month later when tried again.

Dec 06, 2019  After you connect an external hard drive to your Mac, if everything is working fine, the operating system will automatically recognize, read, and mount your external drive, as well as show it in the Finder and on the desktop. However, if your disk gives up the ghost just like the users below, the external disk will not be showing up as normal. Installing the Mac operating system on an external drive is the safest way to get a good look at a new OS without changing anything on your Mac. You get the biggest USB C/Thunderbolt 3 drive.

Getting your Mac to load from a USB drive is fairly straightforward. Use the following steps, you can easily set Mac boot from an external drive in Startup Manager, so it’ll only boot from USB that one time.

Step 1: Insert the USB boot media into a USB slot.

Step 2: Turn on your Mac (or Restart your Mac if it’s already on).

Step 3: Press and hold the Option key immediately after you see the Apple logo. Holding that key gives you access to OS X’s Startup Manager. Once the Startup Manager screen appears, release the Option key. The utility will look for any available drives that include bootable content.

Step 4: Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from. Once selected, either hit the Return key or double-click your selection. The machine will start to boot from the USB drive.

Way 2: Set a Mac Boot from USB Drive using Startup Disk

When you use Startup Disk preferences to set Mac boot from an external drive, so it’ll boot from that disk until you choose a different one. Here is how:

Step 1: Go to Apple menu > System Preference, then click Startup Disk.

Step 2: Click the locked icon and then enter your administrator password.

Step 3: Select External drive as the startup disk, then restart your Mac.

What to do if your Mac does not boot from the selected drive

If you see a message prompts that your security settings do not allow this Mac to use an external startup disk, check the External Boot setting in Startup Security Utility, and then allow your Mac to use an external startup disk.

Step 1: Open Startup Security Utility.

Turn on your Mac, then press and hold Command (⌘) + R immediately after you see the Apple logo. Your Mac starts up from macOS Recovery. When you see the macOS utility window, choose Utilities > Startup Security Utility from the menu bar. When you’re asked to authenticate, click Enter macOS Password, then choose an administrator account and enter its password.

Step 2: Select “Allow booting from external media“.

If you want to select an external startup disk before restarting your Mac, quit Startup Security Utility, then choose Apple menu > Startup Disk.

Note: If you’re using Boot Camp in a dual-boot Windows/OS X environment, you may be unable to boot negatively into supported versions of Microsoft Windows XP, Windows Vista or Windows 10 operating systems installed on external USB hard drive.

Make sure disk has been formatted with a GUID partition type

Intel-based Macs support starting from an external USB storage device’s volume that has been formatted with a GUID partition type. If you wish to boot from the drive, it’s important to format the partition as “GUID Partition Table” rather than either of the other two ahead of time when you use that drive as a bootable drive.

Make sure your disk is bootable

Volumes that aren’t bootable and don’t contain a copy of a valid operating system aren’t listed in Startup Disk or Startup Manager. Make sure the external drive you’re trying to start from contains a usable operating system.

Summary: This post introduces 5 solutions you can try when you are unable to mount an external hard drive on your Mac. Also, to avoid further data loss, you can use iBoysoft Data Recovery for Mac to recover lost data from this unmountable external drive first.

External Drive Won T Mount

After you connect an external hard drive to your Mac, if everything is working fine, the operating system will automatically recognize, read, and mount your external drive, as well as show it in the Finder and on the desktop. However, if your disk gives up the ghost just like the users below, the external disk will not be showing up as normal. And if you check it in the Disk Utility, you will find your drive unmounted there.

'I was able to use my 3 TB external hard disk drive to backup my data last week. But now it doesn't mount after I migrate from Mojave to macOS Catalina. I dug around and tried a lot of stuff but still can't mount this external hard drive on my Mac. Can someone explain how to get this drive mounted so I don't lose the data?'

Why an external drive is not mounting on Mac?

There are numerous reasons for the external hard drive not mounting issue. The following are a few of the most common ones:

Hardware and connectivity issue

Most of the time, you can't mount your external drive on a Mac just because the connectivity is not in proper condition. This issue may be affected by a loose fitting, a broken USB cable, the wear on USB ports, etc. Sometimes, such connectivity issues could make your external drives keep disconnecting, or much worse, it would make your external hard drive unmountable eventually. After all, it is essential to have a firm and tight connection.

Corrupted disk

Another possible reason for an unmounted disk may come from the problematic hard drive itself. Wonder how could it be? Well, it is possible that you're unable to mount a drive just because it is corrupted and unrecognizable by macOS. You can recall if your external drive has experienced one of these things: an unsafe removal when you are transferring files, an improper ejection, a sudden power outage, or even a virus infection. If so, your disk might be corrupted and it should be the culprit that you are unable to mount your external drive probably.

Incorrect formatting of the disk

In some cases, an unsupported file system will make your external drive unmountable. It is reasonable because if your Mac can't even read your disk's file system, how can you mount and access this hard drive on your Mac? This factor may not be as possible as the other causes, but it does happen.

Unable to mount external hard drives on Mac, how to fix?

Even though the reasons for a not mounting disk are different from case to case, certainly there are some things you can do to fix this issue. The following are five effective solutions to the 'external hard drives won't mount on Mac' issue, and some of them are just basic checks.

Solution 1: Check and fix connectivity issues

The first thing you can do is to make sure that your disk is connected to your Mac properly. Here are some things you can try.

• Reconnect your external hard drive to your Mac and ensure that it fits the USB port snugly.
• Connect the external hard drive to a different USB port or a different Mac. If you are not sure if there is a problem with the USB port/hub, you can plug a working external drive in to a port/on a hub, unmount and eject it. Then plug the unmountable external hard drive into that port/hub.
• Replace the USB cable or adapter in case that it was damaged or broken.
• Connect it to a Windows PC and see if the storage device is powered on and working.

Solution 2: Check the file system of the external hard drive

Well, if you don't know exactly what file system your disk is formatted with while your external hard drive can't be mounted now, you can go to Disk Utility to have a check if it is formatted with an unsupported file system. Remember that macOS is friendly with APFS and HFS+ and is in a neutral attitude with FAT, FAT32, and exFAT. However, it is a different story if you need to access and write to NTFS file system on Mac. By default, macOS only enables NTFS drive read support but disables write support. In this case, you can download a free NTFS driver for macOS to mount this external hard drive in read and write mode.

Still unable to mount the external hard drive on Mac? Then it should be some problems with the drive itself. You can try the following solutions to troubleshoot this problem.

Wait! Have you recovered your important data from this unmountable drive?

Before you go any further to fix this unmountable hard drive, you'd better make sure you have backed up your data on this drive to avoid data loss. The reason is that when your external drive won't mount, you lose the only access to all your files on this drive. In this situation, if you have important files on the external hard drive, any operations on the drive could overwrite the original data and cause irreparable data loss.

Forget to back up before? You can also use Mac data recovery software like iBoysoft Data Recovery for Mac to recover lost data from the unmountable external hard drives.

iBoysoft Data Recovery for Mac is afree external hard drive data recovery tool that enables you to recover lost data from external drives, especially when a hard drive is unmounted, formatted, inaccessible, unreadable, or corrupted. It's fully compatible with macOS Catalina 10.15/Mojave 10.14/High Sierra 10.13/10.12 and Mac OS X 10.11/10.10/10.9/10.8/10.7. In addition to external hard drive data recovery, it also can recover lost data from internal Mac hard drives, USB flash drives, SD cards, memory cards, pen drives, etc.

Tutorial to recover lost data from unmountable external drive on Mac with iBoysoft Data Recovery for Mac

Step 1: Download and install iBoysoft Data Recovery for Mac on Mac.

To use iBoysoft Data Recovery for Mac, you need to install this software on your Mac. You can just download the installation package and install it with no fuss.

Step 2: Launch iBoysoft Data Recovery for Mac.

After successfully downloading this program on your Mac, you can drag it to your Applications folder to install it, or you can double-click its icon to launch it.

Step 3: Select the unmountable external drive and click Scan button to scan all lost data.

When the Mac data recovery software is launched, all connected storage devices will be listed in the interface. You can see the name, file system, and space allocation of your hard drives and partitions. Locate your unmounted external hard drive, select it, and click 'Scan' to scan for lost data. Two scanning modes are allowed: Quick scan and Deep scan. Quick scan mode saves time for users and is applied when data is recently deleted. Deep scan mode finds more files at the cost of more time, and it's used when file system is corrupted or partition table is lost. Anyway, this software will automatically choose the most efficient way to scan for files according to the disk condition.

Step 4: Preview the searching results, choose files you need, and click Recover to get them back.

With the preview feature, you can check whether your files on this unmountable disk are in a good condition or not. It saves time and money for users. The software will show if your file is corrupted so that you would not waste energy to recover it or pay for this data recovery product.

Step 5: Have a check to ensure you have got all lost data back.

Now you either have a copy of a data backup in hand, or have recovered your important files with iBoysoft Data Recovery for Mac. Therefore, you can go ahead to repair this unmounted external drive without worrying about data loss.

Solution 3: Repair the external hard drive with Disk Utility

When an external hard drive is not working properly, you can use the built-in disk repair tool - First Aid to check and repair disk errors.

Step 1: Launch Disk Utility.

Step 2: Choose the unmountable external drive on the left sidebar.

Step 3: Select First Aid in the top center and click Run.

Solution 2: Repair the external hard drive with Terminal

Sometimes, Disk Utility may not function as desired. Fortunately, you can still try to repair the corrupted external hard drive with Terminal.

Macos External Hard Drive Not Mounting

Step 1: Go to Applications > Terminal

Step 2: Enter: diskutil list (This will list out the available volumes).

Step 3: Check the volume identifier from the table that appears.

Step 4: Enter:diskutil repairVolume [diskvolume identifier]. For example, if the volume identifier of an external hard drive is disk2s, you should enterdiskutil repairVolume disk2s.

Solution 3: Reformat the external hard drive

If yourexternal hard drive can't be repaired with Disk Utility and Terminal, the file system of this drive might be seriously corrupted. The only way to fix this unmountable external drive is to reformat it.

External Drive Won't Mount Mac

Warning: Reformatting will erase all data on the external hard drive, so please make sure you have backed up important data.

1. Launch Disk Utility from Utilities.

2. Click the icon of the unmountable external drive on the left part of the window.

3. Click the Erase function on the top of the Disk Utility window.

4. Complete the related information (name, format, scheme), then click Erase button.

Conclusion

When you can't mount an external hard drive on your Mac, you can try the 5 methods introduced in this post. But no matter which solution you are going to try, to avoid data loss, you are always advised to back up your files or recover your data from this unmountable drive first with iBoysoft Data Recovery for Mac.

But if unfortunately, none of these solutions can make your hard drive work again, or your disk is not even detected or showing up in Disk Utility, then this drive might be physically damaged. Under this situation, you should send it to a local disk repair service and see if they can help.

Good luck!