What exactly is my unmounted external Time Machine disk doing?

Here is one possibility, but not sure it applies to you. If an external drive is failing or the file system is badly damaged, when it is connected, the Mac may try to run fsck (file system check) in the background, having detected problems, before mounting the drive. In principle, this can, if allowed to run to completion, fix the file system problems. However with a several TB drive, especially one that has problems, it may take some hours to complete, or even more than a day. Activity Monitor may show fsck (or one of the variants on fsck, such as fsck_apfs or fsck_hfs) running in the background. When this happens, the disk is kept busy but does not actually mount until fsck completes. You can tell this is happening by running Disk Utility which should show the drive but the volume(s) not yet mounted (greyed out). In severe cases, Disk Utility may freeze, which means the drive has failed (or is close to failing).

In my experience, once this starts to happen, the drive has reached a state of failure that is so severe as to be unrecoverable. You may have to let this proceed overnight until the drive actually mounts. Sometimes then the drive is very slow or even freezes the Mac, more signs pointing to a failed device.

Before condemning the drive (it might be fine, there could be some other problem), I would shut down the Mac, disconnect all peripherals, then start up the Mac in Safe Mode. That may take 10-20 minutes, fsck is running in the background as well here but this time on your internal drive. Once that process completes, reboot normally. Then when booted up, connect the external drive and watch what happens. It might be fine, or you may see symptoms like I describe above.

In the meantime, get a new backup drive and tell Time Machine to continue to back up to both drives, it will try to alternate but will at least do backups on the new one, while you figure out the issue with the old external drive.

Another tell tale sign of a failed drive -- it won't mount on a different Mac, same delays, slowdowns, not mounting etc. but with disk activity, etc.

Chris Jensen1 wrote:

> If an external drive is failing or the file system is badly damaged, when it is connected, the Mac may try to run fsck (file system check) in the background,

The iMac was restarted, the external drive won't mount, and its drives are very busy (based on the drive activity lights, which are flickering away). A disk check would make sense, but I see no fsck processes when I look in the terminal.

> once this starts to happen, the drive has reached a state of failure that is so severe as to be unrecoverable.

I would be more confident in a diagnosis of a failed drive if there were some error messages, log entries, anything at all, that spelled this out, e.g. "can't mount device", or the like, but there is nothing. Just a non-mounting drive, which is very busy.

Thanks for your reply.

What does Disk Utility show? I am expecting it will show the Time Machine volume as unmounted, while the physical drive may show up, but you need the volume to be functional. If it does show the volume, try First Aid on it. You can try First Aid on an unmounted volume, that SOMETIMES works.

Also, try About This Mac, System Report, and check your USB or Thunderbolt or whichever port is be used for this drive, does it provide information about the physical drive?

If you have other drives that mount correctly and the issue is just with this one, then I doubt you can rely on this drive going forward, I would replace it. If you have another Mac, you can see if it will mount on another Mac, but the drive seems questionable based on what you describe. If the drive is damaged badly enough, it may never get to the point of running fsck in the background.

You can also download DriveDX and check the physical health of the drive (SMART status, temperatures, error conditions, etc.). For external drives DriveDX requires that an system extension be installed first.

Chris Jensen1 wrote:

> What does Disk Utility show?

At the time, it just showed the drive as unmounted. Eventually, whatever check was running on the drive finished, and everything appears normally now. I'm still frustrated to be unable to tell what my external disk is doing after a reboot like this. It's "busy", and I could make educated guesses about what's happening, but I want more information than that.

At the time, I could't find an fsck process. Does a disk check run under some other name after reboots?

Under older MacOS versions, one could see "fsck" running on the disk in the list of active processes. It may be that it is different under Big Sur. I have only seen this once, when a Time Machine disk was failing, and it was under High Sierra (10.13) so that's an older OS and I don't have much experience with it. I would check that external disk carefully, as when it happened to me that one time in the past, the disk was indeed failing and we replaced it.