OK, here is a brief breakdown of the /etc/fstab by column.
device mount_point type options dump
/dev/hda1 /mnt/windows vfat defaults 0 0
/dev/hda2 /boot ext3 defaults 1 2
And so on...
You can copy the /dev/hdxx lines from the fstab of Mandrake into the fstab of RedHat and visa versa.
Just make sure the mount points do not conflict with each other.
If you are still unsure about fstab, post your Mandrake and RedHat fstab and your partition table information. ( use fdisk -l ).