Equallogic PS4000 has 3 spares

We have a PS4000E array model 70-0120.  The array is running RAID 50.  Earlier this week I started getting alerts on drive 2 so I replaced it with a new drive and now the array is report the drive as a spare and not as online as it was before replacing.  The drive that was alerting was a 250 GB and I replaced it with another enterprise level drive that is 1TB.  I haven't had a problem in the past with using larger drives than the originals.

The array now has 15 drives, 3 of which are spare status.  Why would that drive not go back to online?

I can provide more details if needed