This is a wild thought "from the hip" and may be spurious....
You do not say whether Win 98 sees a 20GB disk [across partitions] or a whole 20GB partition.
I know that Win NT cannot handle FAT 32, which Win 98 can use as well as FAT 16.
If 12GB of the disk was formatted FAT 32 and 8 GB FAT 16, it could explain why Win NT can only see 8GB.
I know that an IBM Thinkpad [W98] that I used came with C: formatted as FAT 16 and partition D: as FAT 32. To take it it Win NT required a low level format.
Apologies if this reply is irrelavent, but bets of luck.