Don't buy this if you want a neat fit under your iMac 21.5. If you are not bothered about where you put the drive then it'll probably be great. But, look at google images for the Mac Companion. They show the drive sitting squarely and neatly on the metal foot of the support arm of the iMac. Look more closely - you'll see its not plugged into its power adapter or for example the firewire 800. When you do that, the drive does not fit neatly on that metal foot! In fact, if you are not prepared to force the plug and wires, but want them relaxed rather than stretched or bent tightly, the drive has to sit at least 5 cm forward of the metal foot. If you are prepared to put the drive elsewhere then it would be good. But if like me you want it to actually look as brilliant as the iMac and where all the web photos show it, it doesn't go there. If right angle power and right angle firewire plugs were supplied it might. Maybe if I had a nice 27 iMac it might!
I cannot comment on the operation of the drive as I've only plugged it in and had it far forwards - and decided not to actually use it for data, because of its poor physical fit. Just packaged it up for return. It is though, when spinning, far far quieter than my now old 'MyBook 160Gb'. If I get the refund, I'll look for a drive I can put under the desk.
MyMonster supplied the drive very quickly in excellent condition. Wouldn't fault them for that. However, they have not replied to email sent through the amazon system - even after giving them a couple of weeks more than the required working days. I've had to submit an A-Z claim.
Amazon are my web seller of choice as they have so far been faultlessly customer centred. My iMac is brilliant!