Problem duplicating my external HD onto another one

when I tried to copy my (music files) HD onto another HD, it failed the same way with 3 different 'receiving' HDs: at some point in the process, it stopped, messaged that it couldn't complete the task because "________ (one of my music files) is in use", but I wasn't playing anything. it's no biggie; I had another copy, and can still use the problem HD for playback (it's even still letting me write new files onto it). I'm curious if anyone here has run into this or has any ideas. I'm a mac user, did 'disc first-aid' on the problem HD (it said it's fine). at some point I may just erase the problem HD, (try to) copy the whole archive onto it, then see if it'll allow itself to be copied, but if you have any ideas, I'd appreciate your input. thanks.

fwiw: the file that supposedly was 'in use' was different each time, so I think (?) that rules out that there's a corrupted file on the HD. not sure.
Can you copy the files in batches instead? You might be running into a memory issue as it tries to do a large # in one go.
New to me, never heard of a problem attributable to a source drive. My money is that it's most likely downstream starting at the connectors.
I cleaned mine but still needed a dock. 
@noromance breaking up my itunes library would be a major PIA, and since I'm able to do this action with other drives, not worth the trouble
@fuzztone I was able to transfer the files using a different source to 2 different HDs, each of which has its own cable. it just seems like I've isolated the problem to that one HD
thanks for all the ideas!

I used have this happen when copying a large library from one HD to another. Do not use the windows tools to do this, I use terracopy. It rarely fails with such large IO operations. I have about 2T. If you stick with the native windows tools usually can get it to finish by breaking the cooy operation into smaller chunks. Then when one fails not all is lost. Even in terracopy I do it in 3-500gb chunks just to be safe. 
@jbs thanks for the suggestion. the reviews of the mac version of teracopy are really negative, but I'll definitely check out more mac-friendly applications that do the same thing.