Write speed dramaticly decreasing at half capacity

[gPlusPlus]

Hello

My friend is working on (editing) a feature film and was lent two 5TB external Seagate Hard Drives (one is a backup). They were just purchased by the studio and have never been used before, he is running them on usb 3.0 and not in any raid configuration. He is also running a Hackintosh OS so its not a Windows problem.

He was having no problems until one of the drives filled up to 2 TBs when write speeds started to drop to unbearable speeds, what should be taking 40 minutes is giving an estimated time of 2+ hours until halfway through he simply gets an write error.

He came to me for help but I didn't have an answer.

Thanks

Edit: He is having the same issue with the other drive.

 

[Mattios]

Is it only happening with one of the drives? Could he try doing the same file operations to the other to see if the issue only affects one drive or if it is over the two?

 

[gPlusPlus]

Is it only happening with one of the drives? Could he try doing the same file operations to the other to see if the issue only affects one drive or if it is over the two?

Sorry, I forgot to mention that he is getting the same speed issues with the other drive when he move data over.