Manually put an external hard disk to sleep

Someone reported a similar issue on an Apple discussion forum:

I tried that but the drive will still eject itself randomly at times.

I contacted Western Digital about this and they say it might be a problem with my USB cord. So they're going to send me a new one.

Has anyone else had this problem and found that it was due to a faulty USB 3.0 (compatible with USB 2.0) cord?

Another poster in the same thread commented:

Sounds like disk is on the way out. I would move or backup all data on it.

You should first back up that drive before anything else. Contact WD to see if they can send you a new USB cord. It's a good idea to run some diagnostics on the drive to confirm that there are no disk errors. Testing on another computer would also be a good idea to narrow down to see if the issue is isolated to your machine or the drive.

If the drive is failing, you'll need to return/exchange it.

It's one of the major flaws with newer seagate drives (alongside no on/off switch so you have to pull the power cord out the back and risk damaging the disk). They are just not compatible with Macs unfortunately. They go to sleep and there's no way for the Mac to reconnect to them. I've contacted Seagate about this and they said "Sorry there's nothing they can do about it, perhaps I can contact Apple and they can do something about it". Great..!

I basically don't buy Seagate any more and don't recommend Seagate to any friends or family with Macs. They just cause too many issues.

I'm using 10.6.8, I've not tried 10.8.3 as poster above suggests.