Cannot erase drive with Disk Utility - mount/unmount problems??

Have you tried to immediately erase the drive again after receiving the error? I've seen this problem from time to time for the last year or so. Usually if I attempt to erase the drive again immediately after the error, the process will complete. I always erase the whole physical drive like @P. Phillips' mentioned.

There may also be a problem with the connection, cable, or even the drive itself.

livin1965 wrote:

Thanks. I do not have a Windows computer to try NTFS formatting.

I do have Parallels installed... perhaps I could just use that?

That would probably work as least as far as @ku4hx was suggesting.

When I think the partition table/formatting becomes a problem for macOS, then I just write zeroes to the beginning of the drive in order to destroy the partition table which makes the Mac think the drive is completely empty. To do this requires using the command line. First use Disk Utility to get the drive identifier ("Device" as shown in the Disk Utility screenshot above -- which in the screenshot is "disk2" for the SanDisk at the time of the screenshot) for the physical drive you want to erase. The drive identifier can change each time you mount the drive so make sure to get the current drive identifier just before using the command line.

The following command will unmount all volumes on the drive (change the "diskX" to reflect the proper drive ID such as "disk2" in the above screenshot):

diskutil  unmountDisk  diskX

Now write some zeroes to the beginning of "diskX" (again replacing "diskX" with the correct drive identifier):

sudo  dd  if=/dev/zero  of=/dev/diskX  bs=100m  count=10

This will prompt you for your admin password. Nothing will appear on the screen as you type the password. Press the "Return" key to submit the password.

Now use Disk Utility to erase the drive as usual using GUID partition and the file system of your choice depending on how the drive will be used.

I highly recommend first disconnecting all other external drives (or devices which appear as "drives" such as iPhones & iPads, etc.) so that you don't accidentally erase the wrong drive.

Also try disconnecting all other external devices in case one of them is causing a problem or interfering. I know at one time a Wacom tablet would prevent some external drives from mounting or working correctly with macOS 12.x Monterey.

Also, try erasing the drive while booted into Safe Mode in case some third party software is interfering (anti-virus apps, cleaning apps, and third party security software are the most likely culprits). You can also try booting into Internet Recovery Mode using Command + Option + R to see if the drive can be erased which would eliminate software being the problem.

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

Perhaps two things to try

1 - The method already attempted Except Partition the Drive. Yes, Partition will remove a Partition Table on the drive and and lay down a new one

2 - @ HWTech brings up a good point. The Method of connection to computer. Best and most reliable is a Direct connection and no USB Hub involved

livin1965 wrote:

I do have Parallels installed...

It could be that a process is left over from your Parallels Windows running that is interacting with the drive. This could produce the "busy with another application" message.

There are other background processes that can interfere. For instance, do you have any external drive manufacturer's utilities or firmware (e.g. to lock the drive, or do backups) installed? Do you have any security or anti-virus packages installed? Even such programs that have been removed or deleted sometimes leave startup or other processes and it can be quite laborious to completely remove every remaining piece.

Try downloading and running Etrecheck and paste the output here with the Additional Text button below. Etrecheck provides information about what is installed and running, but no private info is included (you can check this directly yourself before pasting the output).

This happens to me ever so often. My fix is to reformat to NTFS on our old Windows box then reformat it to an Apple format on a Mac.