While running the redhat install program... you should come to a point where you are going to need to partition your drives. On the first screen dealing with this, you are going to begiven three choices.
Manually partition with Disk Druid
Manually partition with Fdisk
Choose the second one... Disk Druid. The other two will mess ya up big time. Auto, because it will rewrite everything and Fdisk because you really have to know exactly what you are doing to use it.
When you get to the Disk Druid screen, your screen will be devided into top and bottom sections. The top section will show you your drive and partitions that currently exist on your system. hda thru hda* will be anything on the primary master drive. hdb thru hdb* will be the primary slave. hdc thru hdb* will be secondary master and hdd thru hdd* will be secondary slave.
Beneath this listing there will be as series of buttons with choices like 'add' 'edit' etc...
Okay, select the drive on the top that is your target for linux... sounds like that is hdb. When that is selected, click the edit button. Then divide up your drive into the slices that you want. When you are done with that, you'll get back to screen with all your drives and partitions listed. You can review everything, to make sure you didn't screw up, and then you can go onto the next screen where you will format the drive... this is where all the changes are made...
Next continue on with the istallation.
For a comprehensive guide to installing Redhat.