Will it do any harm to cannibalise an old hard drive as an SSD enclosure? I have an old G-Tech Mini portable hard drive (FW800, bus-powered) that I don't need any more, so as an experiment I took the actual hard drive out and replaced it with a 512GB Crucial M4 SSD. I would like to use it as an external boot drive over FireWire 800 to speed up an old 2007 iMac.

So far, it seems to be running great! The drive shows up and is nice and fast. But I'm worried that this isn't a 'proper' hard drive enclosure - it could be that it was only designed to work with its original hard drive. I have no idea whether the SSD takes more or less power than the original drive. There is a very slight whining sound when the drive operates, but I can't remember if that was always there. The G-Tech drive was a model from about 5 years ago.

I know that with FW800 and without TRIM I won't be getting the best performance out of the drive, but I'm not worried about that. I just need to know that the chassis won't do any physical damage by delivering the wrong voltage or whatever. Am I doing any harm to the drive and/or enclosure?
The SSD draws a lot less juice from your power supply and doesn't produce as much heat as a spinning hard drive.  You could say your enclosure is quite over-engineered for your SSD.

I would be concerned if you'd try the opposite (putting a spinning harddrive into a SSD enclosure).

That being said it would be better if you'd put the SSD directly into your iMac.  Or swap the drives.  It's not that difficult and you'll probably gain quite some speed.