I would not suggest tinkering with the data cable for PC, for gaming systems it is different, most of the stuff is Read only. Regarding Greg worry of would would happen if u moved the switched during a session.? First keep in mind by togling the switch you are not togeling the OS. the OS was loaded from drive X and now you have toggled to drive Y. The OS will find that the Hardrive is gone. The Hardrive didnt had enough time to pack up (write back) the critical data , so the chances are that you will end up with a corrupted Hardrive, lost data, or even a damaged drive. That is why those Array Controllers with hot swap drives are quite expensive, but still if one pulls a non faulty drive out in those hi end systems, the system doesnt have time to mark it as faulty , so if one put's it back it may / may not work, a new drive has to be put in to rebuild the data from the parity drive. Well the data is not lost in those systems, systems are not interrupted, but the drive is lost. So make sure u dont toggle the switch when your system is powered on.
Go to Ebay and do a search for a "Romtec Trios". I paid about $20 for mine. My home computer has 2 hard drives. One for kids and one for the parents. This switch keeps both hard drives completely separate. You can add a third if you have the room. I can push any of the three buttons while the machine is running but it will not switch to a different drive unless I power down and re-boot. Files from one drive cannot be moved to the other drive(s). It basically makes two computers out of one. Another solution is a $10 kit where you put both hard drives in a removable tray but pushing the button on my Romtec is much simpler. Hope this helps ie. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=167&item=5200747200&rd=1&ssPageName=WDVW
Yeah I was thinking about getting a pair of removable drive trays, one to hold the disconnected drive and the other to hold the drive I'm using. But I liked my switch idea better.
I would still question whether what the OP really needs is a hardware solution (versus a traditional multi-boot software solution) but assuming that he did, I think removable trays are the way to go - they are easy to install, cheap and allow an unlimited number of different boot drives (limited by number of trays and drives you purchase).