"The disk "NAME" can't be unlocked. A problem was detected with the disk that prevents it from being unlocked.

If you need a password to unlock your backup disk on Mac

Do you have the password written down ?

If you back up to an external disk connected to your Mac, and encryption is turned on, then disconnecting the disk may lock it.

To unlock the disk, you must enter the backup password you created when you turned on encryption for the disk.

Store the backup password in your keychain so the drive is unlocked automatically when it’s connected to the same computer in the future. "

Here I am not sure if the Password for the Encrypted TM Backup drive is stored in the Password Application for macOS 15 Sequoia

Use the Passwords app to create, manage, and share passwords and passkeys across Apple devices

Open Activity Monitor in Utilities, View>All Processes, sort on Name, see if fsck is running, if so force quit it.

You can use Drive DX to possibly get a better view of Drive health…

https://binaryfruit.com/drivedx

Badeekea wrote:

X@X ~ % sudo diskutil apfs unlockVolume /dev/disk9s1

Password:

Passphrase:

Unlocking any cryptographic user on APFS Volume disk9s1

Passphrase incorrect or user does not exist

I KNOW that the password is correct.

What does it mean by "user does not exist?" I'm using SUDO.

With all the new privacy & security features introduced by macOS over the last several years, "sudo" no longer has the same God like powers it did before.

However, this has nothing to do with "sudo". It has to do with how file system encryption works on macOS. I found this forum post which may help you understand it a bit better. It is about the only reference that I have found which sort of explains it. Basically when a volume is encrypted, a user must be assigned to that encrypted volume. Sounds like that information or the password for that encrypted volume has been damaged or has somehow gone missing. I've seen a few other recent posts where this error has come up. One case that I recall was where the person could not eject the drive because something was still using it, so they rebooted and could no longer unlock the encrypted volume & received the error you mentioned. I believe another one may have been due to a system update (not sure on that one).

https://apple.stackexchange.com/a/409325

See if there are any firmware updates for that SanDisk SSD. I recall one of the older models having a severe firmware bug which caused data loss on the SanDisk SSD. WD/SanDisk offered a firmware update to fix a related issue, but never mentioned the data loss issue. Some suspected that other models were affected as well.

https://support-en.wd.com/app/firmwareupdate

Were you using the macOS/Time Machine encryption for the TM backup or were you utilizing the WD/SanDisk proprietary software to manage the SSD's built-in hardware encryption?

I did just recently see post where the user was unable unmount/eject their TM backup drive so they rebooted the system. Afterwards the TM backup drive could not be mounted (I forget the exact error). With macOS encrypted volumes can be difficult to figure out. In my own personal experience supporting my organization's Macs, I have found that when an encrypted external drive will not "unlock" & mount (using Apple's encryption features), the volume is actually unlocked, but it is just not able to mount due to an APFS file system issue. Unfortunately the GUI interface & utilities don't let you know this...many times it appears as if nothing happened.

You may get more details by using the command line to unlock the encrypted volume and trying to mount it using the command line (perhaps even in read-only mode without a file system scan). It is not easy to provide instructions since there are so many variables unless you are already familiar with using the command line.

You can try launching Disk Utility and clicking the "Mount" button to see what happens. Then check to see if the volume was unlocked by clicking on the volume on the left pane of Disk Utility and trying to run First Aid on it. You should run First Aid on the hidden Container. You may need to click "View" within Disk Utility & select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility.

Try disconnecting all other connected devices. Connect the TM backup drive directly to the computer.

Try rebooting the computer. Also try booting into Safe Mode to see if that makes any difference.

If the password was saved, then check the KeyChain and delete the password for the external drive from the KeyChain so you are prompted for it once more.

In the Past, did it Require you to Manually Type in the Password ?

What exactly changed between Before and Now ?

To Test if this Drive is Still Password Encrypted

Attach this drive to another Apple Computer that has never seen or used this Drive

Does that computer Require the Password to access the files ?

Having the same issue. Anyone know if there is any known issue with Sequoia? The drive was working fine on Friday but then stopped working this morning. I restarted my computer and now the drive (it's a SanDisk 1TB SSD) won't mount.

Thanks everyone for the thoughtful replies. I've tried everything - ejecting all other peripherals, starting in safe mode, disk utility, using sudo (I also get the "Passphrase incorrect or user does not exist" error.)

I am using a SanDisk SSD hard drive and I am forced to conclude that - especially since my BackBlaze backup logs show the drive just suddenly stopped working in the middle of the night - that the hard drive just went kaput. Lots of Google searching shows SanDisk SSD drives are apparently pretty unreliable and there are actually a few class action lawsuits.

I ordered a new Samsung SSD and BackBlaze is sending me a restore hard drive to hopefully recover most of the contents of the SanDisk drive. Wish me luck.

Thanks again everyone.

Thanks for the quick reply. Doesn't look like fsck is running...

I'll try Drive DX.