Why am I getting “Backup Disk Not Recognized” error message?

My 27 inch iMac is old but is still working, and I've had the My Book since 2015 also. On the info window it says Format: Mac OS Extended (Journaled). It still has over a Terabyte of space and I just want to have it back up all my new iMac stuff. I wasn't using it for this Mac but I need to use it now.

It's properly mounted, and Disk Utility shows no errors, I've restarted the iMac, and unplugged the drive, and even unmounted it and then remounted. But it still keeps telling me that the drive is not recognized. It's visible on the desktop, and I can select it to be the backup, and it appears to be starting. But then stops after about 112 seconds. The task bar grays out and it says "Stopping."

I can buy a new backup drive but we've had a lot of expense lately so I'd still like to use this one if we can. Here is the screenshot of the error message. It has not been erased and the original files are still on it.

Is it just the wrong format?

[Re-Titled by Moderator]

iMac 27″, macOS 12.7

Posted on Mar 20, 2025 12:14 PM

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1 reply

John Galt

Level 10

153,640 points

Mar 22, 2025 01:41 PM in response to LinMcC-MacForever

The backup disk may have suffered some kind of failure. Usually, they can be reformatted and will continue to work for a long time. Or, it may fail again in a very short time.

You might want to wait until you can purchase an additional backup drive before erasing it. Otherwise, you will be left with no backups at all — a situation you never want to confront.

On the info window it says Format: Mac OS Extended (Journaled).

Time Machine has been using APFS since Catalina, and reformatted older HFS+ formatted backup drives, automatically and silently, as it worked. Assuming that Mac is running macOS 12.7 as shown in your profile information, one possible explanation is that reformatting didn't go as it should have. When you eventually get around to erasing it (or you obtain new backup drive and prepare it for Time Machine) it will use APFS by default.

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Why am I getting “Backup Disk Not Recognized” error message?