Many folks in these lists have asked if they can stop Time Machine using all the space on a partition, so they can put other data there too. I grew a partition to make more space and found Time Machine would not use it. This is because it initially creates a Sparse Image with a size set to what is available at the time. Even though the Sparse Image does not occupy all that space until needed, it will not grow past that maximum. In theory it should be possible to fiddle with hdiutil [http://developer.apple.com/DOCUMENTATION/Darwin/Reference/ManPages/man1/hdiuti l.1.html], but that is risky since it is not clear how finiky Time Machine is over this. It does seem to 'fail safe' - that is if anything has been tampered with, it is inclined to refuse to proceed. So try the following:
1) Create a filler file on your volume to occupy the space you do not want Time Machine to use. A (non-Sparse) disk image will do nicely. Check that only the desired Time Machine space is showing as free.
2) Connect to the volume from the machine you wish to back up via a network connection and start making a back up. Time Machine will create its sparse image, hopefully with a maximum size of the space that was available.
3) For speed, if you can connect directly to the volume, stop the backup and connect directly via USB/Firewire and complete the initial backup faster that way. (You need to select the backup disk again.)
4) Delete the filler to release the space you want to use for other things.
I haven't followed this exactly myself, but I am pretty sure it will work.