Is my HDD failing?

I’ve spent the past month backing up my kids photos from my iMac to an external hard drive. My imac is dying a slow death so I wanted to get it all backed up. About 80% finished, it is taking a full day to copy one event through iphoto library manager, as an example.

I opened disk utility, verify disk. the first error was
The partition map needs to be repaired because there’s a problem with the EFI system partition’s file system.

Tried to repair and it said 'Error: Partition map check failed because no slices were found."

I’m at a loss here. Literally my only copy of every photo from the past 12 years (about 90,000 photos) are on this hard drive that is now dying on me. Any ideas?

2010 imac, 2.8ghz, 12gb memory.
Running OS X 19.9.5 (13F54)
WD 3TB external hard drive, 2 years old tops

Disk Utility will run a file system check, but a more comprehensive test would need a separate application. Try browsing the app store for a hard drive diagnostic tool. You’ll need a program that can check your hard drive’s SMART status and scan for bad sectors. I would recommend WD-developed applications but you would need to clarify the model number of your unit for compatibility purposes.

On either case, copying the data to another location should be a priority.