You need to buy a mobile rack or two. They are hot swappable, but since you intend on switching out the OS drive, then hot swappable is meaningless.
- For SATA DrivesVantec MRK-102FD-BK EZ
- For IDE Drives
You may want to buy two of them (of the same model) so you don't have to bother with disconnecting / reconnecting the two OS drives in the inner casing, and then side it into the open bay. Plus doing so can cause wear and tear on the cables.