"Since it's not your "primary" boot disk, I would think that if you bought a replacement and copied everything across, and then assigned the new one the same drive letter, then all should still work OK."
Keef is right.
Do you not have SMART* turned on in the BIOS of your PC? If your disk is failing SMART will notify you on bootup long before you notice any symptoms.
HDTune -
http://www.hdtune.com/ - is one of several free utilities that will tell the SMART status of your disk and various other parameters.
Mac
*
http://en.wikipedia.org/wiki/Self-Mo...ing_Technology