I have a 5 TB Seagate external drive connected to a 2012 MacBook Pro (non Retina) through a USB-3 connector that always gets ejected while I am copying files to it. When I try to copy large amounts of data to it, I get the message
Drive was not ejected properly after about 20 to 30 seconds. I have macOS Sierra installed on that MacBook Pro. Before, I was using this drive with a 2011 MBP with OS X Yosemite, and it did not have that problem.
I have googled this issue, but all I find is posts from people complaining that their drive gets ejected when their Mac goes to sleep, which is not the case here. I can leave my drive connected to my MBP for hours, and it won't get ejected unexpectedly unless I copy files to it.
Anyone else has had that problem? Did you find a solution?
It happens on both USB ports, and it does not happen if I use another drive of different brand with the same USB cable.
I also tried the copying files from a MacBook Air running El Capitan to the Seagate drive, and there was no problem.
If I connect the Seagate drive to a docking station that is connected through a Thunderbolt cable to my MBP, then there is no problem copying the files.
I booted my MBP from an external drive with Yosemite on it, and I tried copying large files to the Seagate USB drive again. There was not problem. So the problem seems to be related to macOS Sierra.