Problem duplicating my external HD onto another one

That is odd.

Sometimes just having a browser (Firefox) window open whilst trying to eject a hard drive can cause issues on my Windows 7 system.

Or perhaps iTunes (or media player of your choice)³ is doing something in the background. Might be worth checking the Apple equivalent of the processes tab.

As an aside, never eject a USB from your car whilst the media system is switched on. There might be all kinds of interactions still taking place and sudden disengaging can cause loss of data.
Or even worse, damage the USB beyond repair!

As I've found out a few times, it's not the same as playing CDs.

@fuzztone: I tried to copy it to 3 different drives with no luck. I was able to transfer to one of those drives using a different source, so I think it's the HDD I'm trying to copy. it's solid-state, if that tells you anything.
@cd318: I didn't have any applications on when I attempted the transfer. the error message says '_________ file is in use' - since all the files on this HDD are music, that would suggest itunes (my player) is on, but it isn't.
fortunately, as I mentioned, I had a different backup, didn't lose anything. I ordered another SSD so I'll have 3 (not counting the problem SSD which, even though I can't copy it, still has all my music).

Are you using all the same Manufacturer for all of those drives?

Can you skip the file and continue?  If so make note of the file and its location and then move it manually after mirroring the drives.

@tom6897 the problem SSD is western digital. I had another copy on a seagate HDD. I was able to transfer that copy to a 2nd western digital.

@danager I can't skip the file; it automatically aborts, doesn't give me that option. each of the 3x I tried to copy the disc, the problem file was a different one; on one attempt it got to ~80% done before aborting, another time <10%.
thanks for all the suggestions. at some point in the next few days, I'll  erase the problem drive, see if I can copy my files onto it and then see if I can then copy it onto another HD. even if it behaves at that point, I'll consider it a HD with an 'intermittent' issue, i.e. unreliable. with storage so cheap these days, I won't risk losing my music collection. at that point the problem HD (even if it's behaving) will just be a redundant backup (I'll have 3 copies of the collection elsewhere). I guess you could say it'll be a lightweight paperweight with a ton of music in it?