ASR Recovery

I use to think a disk is a disk is a disk. I performed a ASR before on XP SP2
before with no problems but with the same vender disk .. i.e. the same model
# and close SN# .... but now I'm trying to restore from a WD SATA 160 to a
Seagate SATA 160 and get the following from restore

Capacity of the replacement hard disk is insufficient, cannot be used to
recover .....

On paper and XP see then as the same size ... actually the Seagate was 100k
larger ... BIOS sees it as a 160 SATA disk it's partition and formated ...
and before you say anything I tried it unpartitioned ... unformatted and the
same thing ...

Do you really need the exact disk for a ASR ??? if so that really sucks....
If not then what's the probem ???

thanks in advance

Dale G