I have 4 1TB hard drives that I'm about to put in RAID 5. One of the drives is defective to the point that it isn't usable. I have a spare 1TB drive, but it runs at 5900RPM as opposed to the other drives that run at 7200RPM. Can I create the RAID array with the 3 correct drives and the slower drive, then when I get my RMA (no telling how long that will take) swap the 5900rpm drive out. Will the array as a whole run at 5900rpm, if it does will it continue to run at the slower speed even when I swap it out with the RMA drive?
First of all, it's almost always better to target RAID 10 rather than RAID 5. It's faster and safer (at the cost of some disk space).
As for your rpm question, yes, the slower drive will slow down the whole array, if your controller even lets you create the array at all, but it's not quite as bad as if all drives were at 5900rpm.
You will also have to suffer through a rebuild when the new drive arrives. Really, you'll be much better off if you can get a rush put on your RMA and wait for the new drive to arrive.