LS220D replace disk

[Springy]

Hi,

I'm trying to replace a failed Disk2 in my Linkstation.
The new disk formats ok but the interface will not let me select the disk to recover the raid.

The full model numbers of the disks are slightly different and the drives are of slightly different capacities.
Can anyone tell me if the difference between the two suffixes and whether or not this is the cause of me not being able to use the disk?

Edit: Can't attach files.  The model numbers are ST3000DM001-1ER1  (2779.8Gb, Disk 1, good) and ST3000DM001-1CH1 (2778.4Gb, Disk2, bad)

Thanks,

Neil.

[nafcom]

Both drives have to be of the same size to re-build the RAID1 Array.