Unfortunately processing going well, not stable

Since I unfortunately due to my professional activities often copy / move large data packets have to (200 - 500 GB at a time), it seems here to come to their limits.
After a good 75% of the copying process, the message that the hard disk was not ejected properly, and the operation is canceled.
As this usually happens after about 2 hours and the copied data is lost, at least on the external hard drive, is a really simple task to a grueling and time-consuming procedure.
Of course I tested this copy operations both on an iMac, as well as on a MacBook Pro, where the error can be reproduced each time, regardless of computer or type of data to be copied. Even reformatting in exFAT brought no help.

Certainly, the hard disk is on a Mac, "just" 50 GB are copied at the their money value, but it is useless for my purposes, and I will send them back to the Exchange once. Maybe I caught only a Monday product.