It sounds as if you could probably accomplish what you need with one or more fixed internal disks configured to allow you to boot into whichever OS / configuration you require.
This would obviously obviate the necessity to stick with IDE / PATA drives, although perhaps somewhat trickier to configure than your current arrangement.
Do bear in mind that while you would be able to re-use the existing disks/caddy/holder in a new PC (that supports IDE), you would still have to completely re-install the OS on each and every disk to work with the new PC, together with all the applications and data on it.
SD