If you have a software mirrored RAID set up be sure to check it every once in a while. Since the data is mirrored you won't always know when one of your drives is sick or dying fast (if there's no noise, that is).
Hit Command + Shift + U to open the Utilities folder and tap D to highlight Disk Utility (you may need to Down Arrow to highlight it) then Command + O to open it.
Once it's open, highlight your RAID then click on the RAID button on the right and hope you don't see this:
If you do I recommend trying a few things first:
Reboot and check it again.
Unmount the RAID, Remount it then Check again.
If it still shows a failure, copy the good drive's data to another good drive and toss out the iffy one. There's no reason to try and "fix" it. You can never fully trust it again and this is the very reason you set up a RAID to begin with anyway - that if a disk fails you still have your data. So get another good drive, add it to the RAID and hit rebuild.
Just keep in mind that when you rebuild a RAID it takes a LOT longer than if you just copy the data over using the Finder. Plus if you rip the HD apart you can yank out the cool neodymium magnets inside and marvel at the technology.