PreDesktop Area
Also known as the HPA (Hidden Protected Area), this is a special area on your harddisk, usually hidden to partitioning tools, that includes all the software and data needed to recover the preloaded state of the ThinkPad.
The PreDesktop area was introduced with the R/T/X 40 series of ThinkPads and is present on the preinstalled harddisks of all ThinkPads shipped by IBM since then.
How to remove it
There's a BIOS setting in my T42p. After disabling the PreDesktop Area, it's possible to remove the partition with standard tools. i.e. fdisk, mkfs, ...
