You mentioned that they're NTFS, which I took to mean they'd been in a Windows machine at some point even if they weren't the system drive. If they were taken from a system that hadn't been shut down properly, I imagine, though don't know for certain, that there's a chance that might cause an issue for some files that are flagged as in-use by Windows.

I'm basing this on an experience I had trying to move an NTFS partition on a drive, which isn't the same as just copying, I grant you, and Linux resolutely refused to do that because it detected that Windows wasn't done with the drive. It didn't say that specifically though, only that there was a problem. It took me a few minutes to figure that one out.

So, if you were to have trouble copying files, it would be worth a try to mount that drive under Windows, do a quick scandisk from there and then unmount it, which ought to clear any active session flags that might be on it.

Again, this is all in theory, because I've not been in your specific situation.

(I did once successfully manage to scandisk and defrag an external NTFS HDD by doing hardware pass-through to a Windows VM running under Linux, but again, that's not quite the same thing, and that was for maintenance purposes, not as a prelude to backup.)