external drive seen but won't mount


Hi. I've got an external USB 250-gig hard drive that is partitioned into two sections. One section I use for short-term storage and am frequently putting on and taking off data from it. The other is for longer-term storage and I only add to it once in a while.

Recently I began to think that I must have a lot of fragments on the short-term side from adding and trashing all those files, and I decided to erase that partition in order to have a clean slate. Using Disc Utility I erased just that one partition, and then added some new data on it. Didn't do anything to the other partition, and it showed up as mounted the whole time I did this.

Now when I connect the external drive I am able to see the partition that I erased and added data on to , but I'm not able to get to the other partition. It shows up in disc utility, but it won't mount. Also, neither disc utility nor techtools will repair the disc or mount it. The partition shows up in the lefthand column in disc utility but it is greyed out. Basically what I see in that column is:

backup 1 (this is the greyed out partition)
backup 2 (this one mounts and I can get to it)

Any suggestions? I looked around in the forums but didn't find anything that quite fit this problem.