As you have explained clearly you have 2 separate physical hard drives, but at the moment you are swapping the cables over.
You do not need to do this; And you can connect both hard drives to the motherboard then enter the bios on boot up by pressing F1 or ‘delete’ or whatever option you are given in your bios screen and choose which hard drive to boot from under boot options. That will then be your boot drive until you change it back in the bios if you choose.
The hard drive you boot from will be your operating system, the other drive will show up as a collection of files, that do not effect the operating system on your boot disk. You will still maintain the functionality of your dual boot system on the single hard drive that has Windows XP and Linux Mint on, with the added advantage that you will be able to access these files when you choose to boot from the physical Windows 7 drive.
Hope this helps.