It sounds like -some- of the files (the ones that won't copy) are corrupted in some way.
Or, there could be a problem with the drive itself.
Can you narrow it down to WHICH files are giving you this error?
If you can, can you still OPEN those files in Photos?
Be aware that when you try to copy a group of files using finder, and if one or more of those files is corrupted, the finder may just stop the entire process and present you with an error.
However...
The backup utility named CarbonCopyCloner has the ability to clone/backup data to another drive, and if it encounters "bad files", it won't quit. Instead, it will copy all the good files and present you with a list of the "problem files" that wouldn't copy.
You might not want to clone an entire drive, but CCC ALSO offers you the option to "uncheck" files/folders you don't want copied.
So...
You could try using CCC. It's FREE to download and use for 30 days.
Set it to clone the drive with the Photos Library to another drive.
BUT...
Use the "some files" option and UNCHECK EVERYTHING except the Photos library.
CCC will then attempt to clone the Photos Library to the other drive (and nothing else).
IF CCC encounters bad files during the copy, it will give you a list of those files.
You can then go down the list and check each one individually.
IF the "corrupted" files will open in Photos, I'd "export" each one to a NEW file, with the hope the corruption will be "left behind" in the process.
No promises, but this might work.