With 100GB ATA drives going for what Zip disks used to sell for, it stands to reason people want an external Firewire/USB enclosure that you can just slide an ATA drive into and eject it from easily. But I can't find one.
Bytecc makes a IDE to USB 2.0 adapter. One part is a IDE to USB dongle the other is a power brick with 4 pin molex connector.
A SATA drive can be connected via a long SATA cable hanging out the back of your system and powered by the power supply from the bytecc adapter I keep mentioning.
Or you can buy a removeable 5.24" mobile rack. This lets you quickly add or remove tray mounted hard drives. However the power has to be off when you switch drives.
Here is the clever part. Mount the mobile rack inside a 5.24" external USB 2.0 enclosure. Do the safely remove hardware thing, power down the enclosure, switch drives, power up.
The enclosure runs $35, the mobile racks $10-15 for cheap plastic models.
Or use that free standing bytecc adapter, but inside your computer case connected to the mobile rack system. Again safely remove hardware, power down the drive (by unlocking the tray), insert and lock new tray.
A SATA mobile rack system would be hotswapable without any tricks.
Do not use an external enclosure to connect multiple hard drive. I used mine that way and had to rewire the internal molex connector and replace the IDE cable. I ruined the IDE cable on a 2nd as well.
I had to find a place that sold 2" IDE cables.
So I just bought a naked IDE to USB 2.0 dongle & 4pin molex power brick.
Of course I am connecting drives as part of repairing PCs (drive tests, backups, virus scans).
Can't really see why you wouldn't just do with one big drive in a single external enclosure.