External Drive Ejecting Itself on Write

I have a late 2013 iMac with several seagate desktop drives attached via USB 3. They all used to work great, even through the Mavericks update. But now one volume keeps ejecting itself whenever I try to write to it. Reading is fine, but once I try to move a file to the drive, it fails and Finder sends a notification telling me a "disk was not ejected properly, please eject the disk before disconnecting". Then the disk usually remounts and shows back up in Finder, but sometimes never remounts and needs to be unplugged and reinserted. Console gives these logs as I try to write a file:

I tried reformatting, erasing, and a firmware update on the drive but this issue still persists. Any ideas?

 

I've tried all four USB 3 ports on the back of the iMac, no difference. In fact I have an identical hard drive, but double the capacity, and switching USB and power from one to the other does nothing. One works, and the other doesn't.

No hub, plugged straight into the iMac, and they have there own power supply from a power strip.

There is also a huge thread from 2009 on Apple's support forum https://discussions.apple.com/thread/2151621?start=0&tstart=0. I will have to read through this, but it nothing to do with sleep settings.
I guess this is an issue with the drive itself?

 

Oddly, I have tested it on another iMac with Mavericks and it successfully copied hundreds of gigabytes without fail (same power supply). I don't have any non-Mavericks machines nearby.