If it is working now, then there is no reason to mess with it. You can run an operating system on any hard drive you have attached to your computer, heck there are even hacks to have an ipod bootable.
When Jag came out I was running 9.2.2 on one hard drive and 10.1 on another and 10.2 on another and had no problems switching between the three.
What I do when installing a hard drive in my once had power mac, I would make all of them cable select and then forget which one was on the salve or master portion of the cable. I don't think Macs are too picky about this and now even XP isn't picky. You can install XP on a salve hard drive as well. (well if you own a pc.)