Identical HDs but different size after formatted?

[JimmyNeutron]

Hi,
I have two identical hard drives with same manufacturing date, WD5001ABYS. but there are two different motherboards; one on Intel D975BX2 and the other on Asus Rampage Formula.

Intel MB is running XP 32bit and Rampage is running on Vista 32bit

After initializing and formatting the hard drives, I get the following from each of my HD using CHKDSK

From XP machine:
488318975 KB total disk space.
4096 bytes in each allocation unit.
122079743 total allocation units on disk.

From Vista machine:
488319740 KB total disk space.
4096 bytes in each allocation unit.
122079935 total allocation units on disk.

Both systems are using 4k sector size.

Could someone explain why the difference in size? I understand the differences is very minor, but I'm more interested in why the differences in size on identical HDs.

Thanks!

[[21CW]killerofall]

I think that a 0.0001494140625% variance is within the errors of manufacturing (thats 15 1/100,000th of a %) which is excellent IMO. Remember that a 500GB HDD is actually a 500,000 MB HDD and I took that into account when calculating the %.

[geiger]

It's from differences in the diskpart app in vista/xp. The vista version does automatic LBA alignment that the XP version did not do automatically.