Rebuilding a Promose Raid array on Asus motherboard

Got a problem with the following:

- Asus motherboard (P4P800-E)
- Onboard Promise Raid Controller (Fastbuild Util 2.01)

On boot, the following is displayed

Problem is detected with array : 1
Critical Status:
A disk memmer of a mirrored array has failed or is not responding.
The array is still functional, but fault tolerance is disabled.

However, the Fastrack utils sees both drives (i.e. both are connected and functional).
Also, booting with one (either) of the drives connected, Windows XP boots normally.

So it looks like the array has lost sync.

I have got a new HD of the same type (Maxtor 6L250S0 SATA) but the rebuild function in Fastbuild says there is not enough capacity.  Looking at the original drive and new drive in View Drive Assignments, the original drive capacity is 25100MB capacity and the NEW one is 250059Mb.  I presume the new drive must have some bad sectors, but because its smaller, I presume this is the reason the Raid controler won't allow it to sync.

Three questions:
1. Can I get the Raid array to sync using the old drives
2. How can a new drive with exactly the same model number have a different (lower) capacity.  If due to bad sectors, how in the hell can you buy a NEW drive and be sure its going to have enough good sectors to join the array?
3. If I have to start again and rebuild completely, how do I 'zap' the original SATA drives?

Thanks
SF