Drive not unmounted because it's in use

This has been a problem going all the way back to 2015 in OS X El Capitan (mds_stores prevents unmounting of externa… - Apple Community "). It seems spotlight grabs a hold of the drive to index it for search, and while that is running, the drive cannot be ejected. And it seems to take a VERY LONG time to complete.

IMO this is a bug of Apple's as the unmount command should kill any mds-stores processes running on that drive. But alas, since they have not fixed it in 10 years, I do not hold out for hope that they will ever fix this bug.

Follow the above link and use the steps listed there to kill the mds-stores process. You should be able to unmount the drive, although it make take one or two attempts.

EricWeir wrote:

I have a new WD My Passport external disk that I'm using for Time Machine backups. This morning I checked to see when the last backup was. TM is set to back up every hour but the last backup was late yesterday afternoon.

I tried manually starting a backup. I got "backup failed because the drive is in use by another app." I tried using Disk Utility to unmount the disk and again got "disk not unmounted because it's in use." I restarted my MacBook. Afterward manual start of backup worked fine.

I encounter this situation not infrequently. Including when trying to access other folders on the drive. Why is that? Is there another way get around the difficulty when it occurs? Is there anything I can do to prevent it from happening.

Did you erase reformat and initilize that drive as new...(?) this will remove any proprietary software that may have been pre-installed (esp and drive that says Mac friendly/ WD comes to mind.)

Time Machine is looking for GUID/apfs case sensitive format

Erase and reformat a storage device in Disk Utility on Mac

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Trouble Shooting Time Machine —drive not mounting use Disk Utility.app>First Aid to Repair Disk

Time Machine troubleshooting on Mac - Apple Support

EricWeir wrote:

I'm not clear what GUID is. I assume Time Machine selects it. I put another volume on the drive for my photos.

GUID is a "partition map" (Globally Unique Identifier)

this is different from the the "file format" which is the preferred 'newer' APFS (Apple File System) introduced in 2017.

ref: File system formats available in Disk Utility on Mac

What I can say about your issue— Time Machine likes to have it's own dedicated drive, not shared with other Volumes for a trouble free experience.

Get a different drive for your photos.

EricWeir wrote:

The last two times I let Time Machine itself set up the volume. This time I limited the space that could be used--quota?--to 4Tb. I'm not clear what GUID is. I assume Time Machine selects it. I put another volume on the drive for my photos.

Are these the only copy of those photos? If not, then are you backing them up as well to yet another location besides the TM volume on the same drive?