The Predesktop Area is part of the HPA (Hidden Protected Area). The HPA is a special area on your harddisk, usually hidden to partitioning tools. It includes all the software and data needed to recover the preloaded state of the ThinkPad. The HPA also includes some diagnostic tools and a (MS Windows only) backup tool.
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.
General information about the HPA
The HPA seems to be using Phoenix FirstWare. FirstWare is (in short) an implementation of two technologies: BEER and PARTIES. (Yes, those names are correct!) BEER (Boot Engineering Extension Record) is described in this document. There is an introduction to PARTIES (Protected Area Run Time Interface Extension Services) on the IBM site.
Basically, what seems to be going on is that the Phoenix BIOS hides the last few gigabytes of the harddisk (that is the HPA) to the OS. Note that this is just a setting in the BIOS and can be disabled. The HPA can be accessed by pressing the Access IBM Button at boot time. The BIOS will then parse the BEER and the "Directory of Services" (situated in the last sector of 512 bytes of the harddisk) to see what part of the HPA should be launched. In (most?) ThinkPads the BEER and the Directory of Services tell the BIOS to launch the Access IBM Predesktop Area. The system will then actually be booting into a (minimal) DOS which is able to launch a graphical shell (called Phoenix FirstSight). IBM has simply rebranded this graphical shell to the Access IBM Predesktop Area. From this graphical shell one can launch several tools (BIOS Setup Utility, diagnostic tools, recovery tools).
Details of the HPA
Fabrice Bellet describes a technique he used to explore the HPA of his ThinkPad T40, using GNU/Linux tools. This technique is only for the more curious or more careless people. It uses "dd" to copy the sectors on the harddisk containing the HPA from "/dev/hda" to a new file: when using "dd" on "/dev/hda" you are only one small typo away from an unrecoverable disaster!
Here follows a more detailed description of the HPA on a ThinkPad T41 (60 GB harddisk) to contrast his findings.
On this ThinkPad T41 the HPA is 3,4 GB in size. It contains 7 consecutive PSAs (Protected Service Areas). The first 6 of those start with an x86 boot sector.
- The first PSA is 3,2 GB in size. The OEM-ID of the boot sector is: "IBM 7.1". It seems to hold a copy of the preloaded OS and everthing needed to generate a bootable DVD-ROM for it, even an El Torito boot image and a boot catalog: see Backing up the preloaded OS.
- The second PSA is only 7,4 MB in size. The OEM-ID of the boot sector is: "MSWIN4.1". It seems to be an image of a 1,44 MB bootable floppy disk (with MS DOS) and directory containig 6 MB of FirstWare tools. It will be launched by the "Recover to factory contents" tool of the Predesktop Area. Those "factory contents" should be the data on the first PSA.
- The third PSA is only 1,4 MB in size, The OEM-ID of the boot sector is: "IBM 7.1". It too seems to be an image of a (sort of) 1,44 MB bootable floppy disk. It will be launched by the "Restore your backups" tool of the Predesktop Area.
- The fourth PSA is again 7,4 MB in size. The OEM-ID of the boot sector is: "IBM 7.0". It will be launched by the "Run diagnostics" tool of the Predesktop Area.
- The fifth PSA is also 7,4 MB in size and the OEM-ID of the boot sector also is: "IBM 7.0". It will be launched by the "Create diagnostic disks" tool of the Predesktop Area. It contains a copy of a (sort of) bootable 1,44 MB floopy disks, some tools and compressed copies of the diagnostic disks.
- The sixth PSA is only 1,4 MB in size. The OEM-ID of the boot sector is "PHOENIX". It seems to be a copy of a (sort of) 1.44 MB bootable floppy disk too and only contains a (minimal) DOS and the FirstSight application. Basically, this is the Access IBM predesktop Area.
- The seventh PSA is 101 MB is size. It doesn't have a boot sector. It contains the FirstWare Reserved Area. That probably is some sort of swap space for the FirstWare system.
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, ...
It might be possible to use the FirstWare tools included in the HPA to make the HPA more useful for GNU/Linux purposes. For instance, the copy of the preloaded OS could be replaced with an emergency backup of your GNU/Linux distribution. Maybe the Predesktop area could be even used to boot into a GNU/Linux rescue system. Whether the Phoenix propietary tools really allow that and whether those tools do not make it so hard to accomplish cannot yet be said. It seems realistic to assume that the benefits of those alternative uses aren't worth the effort to accomplish them. Still, it might be fun (altough possibly hazardous to your system) to try ...