Copying .TIB files between USB3 drives

Pretty sure this is a Microsoft issue, but want to cover my bases here.
Using Windows 8.1 Pro 64 on a ASUS P8Z68-V LX
I tried to copy a 160GB .TIB file from a 1TB Seagate Expansion drive connected by USB3 to a 1TB Seagate Barracuda 7200.12 drive in a NexStar NST-D300S3 dock connected by USB3.
The copy window started at 90MB/s (decent writing speed for that Barracuda) then dropped to zero after about 10 seconds. 10 seconds later it went back up to about 50MB/s and then back to zero. This repeats for a couple of minutes and then the copy fails with an error message about the source file not existing.
I can copy the file from USB3 to SATA and then back to USB3 with no issues.
Has anyone seen this kind of behavior before between USB3 devices?
Dan