Problem with External Hard Drive

I have a Western Digital 500gb External Hardrive (USB) and i had it partitioned, half was for backing up, and the rest was for extra storage. Long story short, i the old time machine partition, then dragged the partion thingy down to make all the space go to the one partition so i could use all of it for storage. It did its thing, and then i went to add some more stuff to the drive, and i got the error of not enough space. I went and look, and it had the same amount of space left, and the other partition disappeared. What happened to 250gb of space??? it disapeared!