Large file transfer stops working when using USB hard drive
I am transferring large video files and folders from one drive on a WD My Book Duo to another in the same My Book Duo. Both Drives are WD Red and formatted for HFS+J. The My Book Duo is connected to my iMac. Every time, part way through, it stops. I try to continue by merging, but that stops as well. The windows that were involved in the transfer disappear. Most of the shortcuts and folders on my desktop disappear as well. No error message. I force quit finder and restart the transfer. It does the same thing, sometimes after 300GB, sometimes after 800GB. Completely random.
This problem has been going on for a few days now. Any help?
iMac 27″ 5K, macOS 10.15