At the time of rebuilding the array you have the option of taking out each drive and running it through utilities in another computer for the best testing that is available by software, if time permits. You also might be able to do the same by leaving the signal cables on all drives but leaving on only the power plug on the drive in test on your RAID box and let the controller see a single drive as a single drive. (Some hook ups make this electromechanically hard). Then there is what we could do with old type hard drives not too easily and modern ones with much difficulty, which is, to put new bought hard drives on their backs and go over them when they are running an interleave test, with the probe of practically a DuMont of an oscilloscope and solder on tiny brass loop electrodes wherever we find playback signals after the pre-amplifiers and servos. (If we cross out a connection accidentally we have to buy a new hard drive but at least we have not lost any data). Of course, this is something every magnetic maedia drive manufacturer from floppy and tape drive makers on up should already have provided us along side of a circuit diagram; but let`s face it, the computer crowd are the fast boys and not watching if the manufacturers pull the wool over their eyes. The manufacturers have not taken the time to offer this integrity as a sales pitch of quality either. Leo Laporte on, ¨This Week In Amateur Radio Incorporated¨ sometimes mentions his favorite hard drive testing software but I don`t remember its name.