NT 4 (compatability issue Pentium 4 PC)

Can anyone give me more information on the compatibility issue surrounding running NT 4.0 on a Pentium 4 system (512 RAM, 20GB hard drive).

The situation is such:
install NT 4.0 (partitioning at 4GB)
I apply the service pack 6 and I try to partition the remaining hard drive (15GB)it fails on reboot asking me to insert a startup disk.
I have tested this with two different hard drives and I receive the same results.

HELP!