http://www.win.tue.nl/~aeb/linux/ - A great amount of detail covering disks, concerning ... well, take a look and you'll see. Note a wealth of information is held there, supplied by number of people that clearly know their stuff.
http://www.stud.uni-hannover.de/user/76201/gpart/ - Utility for locating lost partitions.
http://www.sleuthkit.org/index.php - A number of forensic style tools that support NTFS, FAT, Ext2, Ext3, UFS1, and UFS2 file systems.
http://www.geocities.com/thestarman3/ - An in depth look into the Master Boot Record of a MS system see
http://tldp.org/HOWTO/Linux+DOS+Win95+OS2-2.html - A step by step process for building a multiboot compatible partitioning chain.
For extensive information on the design of hard disks see:
http://www.logicsmith.com/anatomy.html
http://www.dataclinic.co.uk/data-recovery/index.htm
For details on File Systems see:
http://homepage.cs.uri.edu/faculty/wolfe/cf/readings.html
http://pclt.cis.yale.edu/pclt/BOOT/IFS.HTM
If anyone knows of a site that thoroughly details / or even at all the structure of a primary extended partition I'd like to know as I have not been able to find any, the first site listed touches on the subject though doesn't take it any further.