Here's what I'd do. I would plop my mandrake disk in. Select the keyboard I want, then the install type (for me I choose expert because I like a lot of choices). Then when you get through configuring your mouse and scsi devices, you should be near partitioning. Depending on which disk you are installing to, replace my 'b' with either 'a' for your primary.
Click hdb tab, click the (probably red) colored bar that represents your Harddrive. Choose delete (assuming you have nothing on here you want). Make a smooth white bar across the top. Click the white bar, and choose mount. Then choose your mount point (I usually specify / first) and the filesystem type (Go with a journaling, either ext3 or ReiserFS). Choose a size (this is up to you, do a search on this site if you are unsure of how big to make each partition --- I usually dedicate most of my partition size to / but that's just me) and then click ok. If you are going to make more mount points, do them, and do similar things as described above. Just to give you an example(not suggesting this is right or wrong): I have /home (as ext3 with 3GB) /boot (as ext3 with 50mb) and /LFS (as ext2 with 7GB). Once you are finished with all your mount points and such you will want to make a SWAP. so mount /swp with the remaining empty (white) space. If you are unsure on how big to make your swap (or to save to make your swap) do a search on here, this is a heated topic and I will not recommend a size on here.
Now that should have taken ALL of the white space and changed it into a dark red or something color. It is important that you have the / and /swp at least. Now you can press 'done'. And continue on with the rest of the install. If you do all the above steps and it still fails, please let us know.
And if I confused you, or if someone has a 'fix' to my response please, let us know. (Us does not mean I am borg, see I said I.)