I have a 2nd hard drive in my computer and I want to know how I could format it in linux. When I was installing Slackware I had red hat installed and it wouldn't format it because it said red hat was useing it. I now have Slackware installed and when I got to system monitor in gnome and check devices /dev/hda2 only shows up. How would I format hdb and put my /home directory on it?

run fdisk -l to see if the drive is actaully there.
run mount to see your current mounted partitions
create partitions on your new drive with fdisk or cfdisk
create a filesystem on the new partitions with mke2fs, mke3fs, or mkreiserfs
mount the new partition temporarily to /mnt
copy the stuff in /home to the /mnt
check the /mnt point to make sure the data copied properly
unmount the device from /mnt
mount it as /home.

If you want specific help you will need to post the output from fdisk -l and mount so that we don't overwrite critical data by giving you incorrect information.