Welcome to LQ;
Boot into Linux
Run "fdisk -l" (no quotes---run it as root) and confirm how the drives are being seen. Let's assume that you Linux install is on sdb and sda is the one you are going to remove. Also verify which partition has /boot on it---let's assume sdb1. **
As root, run "grub" to start the grub shell, and then run these commands:
root (hd1,0) **
setup (hd1) **
Now, when you remove the other drive and start up again, what WAS sdb will now be sda, and you'll boot directly into Linux.
Note the numbering rules: To the Linux OS, sda is drive #1, and sda1 is the first partition on drive #1. To GRUB, hd0, means drive #1, and hd0,0 means the first partition on drive #1