Raid 5 rebuild on power up

[rjes]

My N3200 is on most of the time. I only turn it off if I know I won't be using it for a few days. In use it seems to be very stable. However, the raid 5 array seems to rebuild about 80% of the time when I switch N3200 on.
Each time the raid array rebuilds, it seems to be because my N3200 has detected an error in one of the drives. This worries me, as sooner or later it will detect errors on two drives, and the data could be lost (it's backed up, but I want to understand what's happening).

Can someone explain this:
Are my drives failing?
Is my N3200 over sensitive to very minor errors occuring in my drives which actually don't affect their performance?
Is this normal N3200 behaviour?

One of my three 'identical' drives actually has different firmware to the other two. This drive also has no bad sectors, while the other two do. Should I look in to updating the firmware of the other two drives?

Thanks. The details of my system are below:

N3200 Firmware: 1.00.04

Tray 1:
Seagate ST31000528AS
Firmware: CC44
Status: OK
Reallocated sector count: 0
Seek error rate: 2453043

Tray 2:
Seagate ST31000528AS
Firmware: CC34
Status: Warning
Reallocated sector count: 1
Seek error rate: 5606421

Tray 3:
Seagate ST31000528AS
Firmware: CC34
Status: Warning
Reallocated sector count: 24
Seek error rate: 5449280