You may find the drive being slighly different, may not be supported for this array. I seen theads that go either way but I have always been advised to keep the drive identical.
Particularly true on the older PERC cards - 7 years old, so a 3 or 4?
Or it could be a hardware fault/error on the controller card, too. Either way, I'd replace the controller card, if not the whole server. I've seen a bunch of really nicely speced out PE2950's on eBay for around $2500, which I have to imagine would be a massive boost over the 7 year old whatever you're trying to run now.