Yes, I use LBA as well.
Well, I have solved the problem....
I use the GAG bootloader on the mbr (http://gag.sourceforge.net/)
. One of its "features" is that it hides the primary partitions so you can have multiple versions of DOS insalled; and as a result, GAG was hiding my shared partition.
I have never had GAG do that before because I have always made the shared partition logical (placed it inside an extended partition).
I don't suppose there are many other GAG users out there, but if this ever happens to any of them... here is the answer!
Thanks for the help guys, if for nothing other than getting my brain working.
Edit: For those who may stumble across this later, there are lots of ways to unhide a partition. Both the GRUB and LILO bootloaders can unhide partitions, as well as the parted and fdisk partition tools and more. If you use GAG, just don't expect to use it and be able to access your primary partitions from windows (Linux isn't so picky).