This is an old post, preserved for reference.
The products and services mentioned within are no longer available.
I still have need to test IDE drives, and they are more of an issue as I either have to dismantle the second PC and connect it internally, which means it is present during boot which isn't ideal, or I have to mess around with USB-IDE adapters and external power supplies which isn't ideal either as it is a bit of a faff and is limited by USB speeds. I got a couple of removable IDE caddies which improved things as it made it easier to connect to the PC, but still meant I needed to have the drive present during booting.
So the idea I had was to use a couple of IDE-SATA convertors. These little boards allows you to use SATA drives with an IDE host or, as in this case, use IDE drives with a SATA host. I did some testing with the drives powered up all the time, and connected via the adapters to the SATA ports on the motherboard and they were detected fine and accessible at full speed. All that was needed now was hotswap capability.
The caddies have a key operated power switch, which cuts power to the drives to allow removal. The SATA-IDE adapters only need 5V (usually provided by a 3.5" floppy power connector), so I found the switched 5V line on the back of the caddy and wired it to the SATA-IDE adapter power so it would be switched on and off with the drive.
Now for the test. Power it on before boot to check it all still works, yes, no problem. So now I booted with the key turned off, and once booted, turned they key on. The drive spun up and was detected just as if I had slid in a SATA drive. I then made a second to complete the pair and now use these regularly for testing IDE drives.