I have a laptop with 2 disks that originally were raid-configured. One of them is failing. What is the best way to check for hard drive health and disable on of the hard drives? Can I reinstall windows on one of the drives and use that one only?
Typically your RAID controller should have an interface for you to diagnose such issues. If it's hardware-based you may have to access this from a boot up shortcut key press when you power on your laptop. Sometimes these RAID controllers have a software that go with them so you can control things from inside the OS. If you're using the built-in software RAID controller on Windows you should be able to go to the disk manager to see which physical volumes are degraded.
If you're using RAID 0: Back up everything ASAP or you'll lose everything (assuming you haven't already).
Any other RAID: Just swap out the failing drive with a good one ASAP.