2tb My Book Essentials unmounting during file transfer

I am trying to transfer a large amount of data from one harddrive to another. They’re both “my books”. I have transferred this same data, same amount, from one drive to another before with no issues, but not with this harddrive. It is just a large amount of music. Every time I start copying the files, about one minute into it, the drive unmounts and interrupts the transfer. I have actually transferred alot of data at once to the drive in question before, though I’m not sure if I’ve done quite this much at once (128gb). Is there some explanation for this, like a max amount for one transfer, that I am unaware of even though it has worked with other drives before?

edit: as part of my troubleshooting I updated the drive’s firmware.  I think that actually did the trick, so far so good, we’ll see.  If the whole transfer goes through without a hitch, I’ll post back so that people know this is the fix if they run into the same issue.

edit2: I was mistaken, due to a stupid mistake I made the transfer was not in fact taking place at all.  Will now attempt and post results.

edit3: Firmware update completely 100% fixed my issue.  If you have this issue, just update your firmware.