I've had this problem since I switched to Mac back in early 2007 (Mac Pro) up to and including today (currently running a mid 2011 27" iMac).
Recently I started investigating a problem I've had with my QNAP TS-220 NAS. It's a simple 2-bay NAS in which I have 2 NAS spec'd Seagate drives; each 3TB.
I also have an external HD connected to the NAS via eSATA. The HD is a Seagate 2TB drive.
I use this drive in order to back up data from the NAS via a weekly scheduled job.
Ever since I've had this job set up, I would get a weekly error on the NAS and I would note that the backup job would not be complete. I let this linger for some time. Not paying attention to it much since it was "only a backup job".
But then I got to thinking about my continuing, agonizing, painful and frustrating Mac external HD situation and how the external HD would get "bumped" off for no particular reason, usually after sitting mounted for who knows how long.. sometimes days.. sometimes hours... - the External HD would still show as desktop icons and in the Finder but once you try to access the drive... I get to play with the dreaded beach ball until I forcefully eject (turning off the external HD) - as I said at the beginning of my post, this has gone on as long as I've owned a Mac.
I started searching for compatibility issues with the NAS. My HD's inside were fine because I knew they were according to QNAPS listing. My external; after examining it, was a Seagate Green drive. I had heard that the green drives were probably not the best to be using for lengthy use but for brief access. I then checked the external HD attached to my iMac; it was also a Seagate Green drive. I wonder if the Green drives are what the issue is here.
I've searched through this entire thread and only one other individual actually mentions that this may be the reason for drives being kicked off (I searched the word "green"). I have a single 1TB WD Black drive that I will test out before spending any $$$ on new drives. I can at least test it out to determine if the green drive is at issue with my QNAP job and, hopefully, that will translate over to the issue with the Mac.
Cheers
Dave