I am not sure how he ended up with unallocated space - but the same thing happened to me while using PM-8.
I purchased 2 externals - a 500 gig and a 320 gig. I wanted to partition the 500 into 100's each and the 320 into 80's.
I formatted the 500 gig external then activated PM. The drive for the external showed F, and 500 gigs of space. I chose the first one to be 100 gigs and it did that with no problem, giving it the letter G. However, when I went for the second partition on F, it came up with unallocated space (a lot!) and shrunk the size of G to 80 gigs. When I tried for a third partition, the same thing happened - it shrank the size of the partition preceding the new one.
I tried several times - clicking UNDO and backing out of PM. I called a friend who had partitioned her external and she couldn't understand why it was doing any of that. We tried over and over for an hour before I gave up again.
I took the drives to the guy who built my computer... safer to pay him and get it right.
~M