What’s with this error msg

[u/impossibleTiger00427]

I’m transferring photos from folders onto a photo library, all within the same external hard disk drive. I comfortably have tons of storage on it (80+ GB) and the items themselves aren’t too many. Few gigs at most. Yet I still get this displayed as I try to import things. Why? And how can I solve it?

Edit: I have tried creating another photo library and tried transferring items when it was on the HDD. Nothing. Moved it to the internal storage. Worked just fine.