Never trust a failed disk in a RAID array. Ever.
I had this happen to me plenty of times, I didn't bothering setting up Nagios to monitor the hard drives SMART status and hard drives kept reporting failures (plead guilty for not doing any or not knowing actually) but nothing would happen to the server, it would remain up and working.
Until the hard drive really starts pucking up. The whole RAID array starts getting corrupted instead, happened with 3Ware and Adaptec controllers, then I end up having to fsck the file system with a lot of corrupted files, which seemed like an eternity when the clients are begging for the server to be back.
So I really would recommend just using it somewhere that you don't care about your data, or if the hard drive is not that old, RMA it. Both Seagate and WD have impressive RMA procedures and you usually get a refurbished hard drive, sometimes with even bigger capacity.
However, don't touch Seagate hard drives. I have lost all my trust in them, I have had a good 25% failure rate of those hard drives, from different distributors too. NS or AS version, they all failed. I have been using WD RE3 and it's been great.