What interface is in the caddies? The older ones will be IDE and buying IDE HDDs is a bad idea, newer are SATA3 and you can push over 100MBps through them if using USB3, if you get an off-the-shelf caddie it may not be up to the scratch.

If you could blue-tac the USB cable down or something could you simply use that in place of a power switch? Realise it is slightly less convenient, but I think the benefits of getting a new external drive outweigh this.

Just my opinion though!