Of course, this isn't a very realistic example,
but it's a possibility. A more logical example with four drives would be a
three-drive RAID 5 array with the fourth drive as a spare. The CI/O software has
an activity that tests all spare drives at 2:00 AM daily by default, so you can
rest assured that the spares will most likely work should a drive in the array
fail. You can verify the integrity of an array any time you want, but it will
take a long while with large arrays- about the same time as it took to
initialize the array. Reconstructing an array with a failed drive is also easy,
and you can schedule such operations if necessary.
Lastly, you can pause the I/O on the array, which can give you pseudo-hot swap ability if you have removable drives. Unfortunately, the AAA-UDMA doesn't support true hot swapping, SAF-TE enclosures (since they're external), or S.M.A.R.T. functionality. If you need any of that mission critical type stuff, you'll have to get a SCSI RAID controller.