iCoud Photos

Sorry - no idea, other than it is possible the drive has failed. If this is the drive with your library on it probably explains the missing images.

Might just be worth trying different USB ports on your mac (avoid any hubs between mac and drive). Is your mac able to reformat that drive (Note reformatting will erase all data). If it can't then can you try to reformat in a different machine - even a windows PC. If not it further suggests the drive is faulty.

This article

https://softwaretested.com/mac/how-to-fix-the-com-apple-diskmanagement-disenter-error-49153/

also suggests that error might be related to time machine. Are you using this drive for storing time machine backups - if so that is not a compatible location for a photos library, which cannot share a time machine drive - see:

Move your Photos library to save space on your Mac – Apple Support (UK)

Yes, create a new empty library on the new drive, make sure it is the system library and iCloud is enabled. This should then sync with iCloud. If you library is large this might take days or even a week or two. Leave the mac on, connected to power, with the photos app open.

The drive needs to be USB connected, formatted APFS, or MacOS extended.

If you select optimise mac storage then full size originals will not be downloaded from iCloud - only thumbnails and previews will be created in your local library. But then you need to worry about how you backup your photos.

Optimise mac storage will remove the (full size original) photos from whichever library is synced to iCloud. They will only be stored in iCloud.

If you are storing your library on an external drive, then simply don't select it. It won't do anything for your system drive anyway.

Good news. Though I am always suspicious of a drive that has failed in this way. Make sure any data you store on it is regularly backed up (you should of course do this anyway) 😁