Wondows 7: hard drives are randomly ejected

I hope this hasn't been asked yet:

I'm copying files from HDD D: to HDD E:, and at a random moment, one of them - sometimes both are ejected. Most of the time the ejected drive(s) are mounted again, though. Still, I'm not able to copy files probably.

There's no issue hooking up both drives with the same cables to another computer. It works as expected.

My main drive (C:) is never affected.

The question is now, is the SATA controller at fault? I don't think it's driver related, since my main drive is hooked to the same controller.

Thanks for your help!