Win NT4.0 Harddrive sizing

I have just installed winnt 4.0 workstation. I would like to know why the Install procedure would not allow me to fully allocate my 6Gb harddisk to one drive. I had to partition into two drives which is a minor pain if anything. I tried selectingNTFS and then Fat as the preferred format but was rejected on both, the OS saying the size of the partition was too large.
Is there a fix to this? I have tried using the updated atapi.exe from the MS Knowledge Base and Service Pack 6.0 is installed at present.

Previous OS was Win98 SE, box is 433Mhtz PII.

Thanks