We are setting up a RAID 5 with 4 identical Seagate 750 drives. Now the dummy part. What will happen in a year or two if a drive fails and we cannot find an exact replacement?
Just make sure the relacement drive is bigger or the same size. The RAID will automatically rebuild but will only use up to 750GB or the smallest size drive in your RAID group.
I have done this many times at work. We had 36GB hard drives that were replaced with 70GB hard drives from dell.