What type of enclosure? I have (and still have) two enclosures that won't
take a drive larger than 80 GB, one that limits a replacement drive to 120
GB and I have replaced the drive in a SympleTech enclosure (160 GB original
drive) with a 300 GB WD drive.
You say the drive is formatted to 300 GB. Where was it formatted to that
size? Certainly not in the drive enclosure you currently are using. XP SP2
by default should recognize the drive with no problem.
Go to Computer manager, Disk Management and look at the bottom right window
for the drive in question. What does it show as the drive size and
formatted partition size?