Disk utility partition tool shows incorrect free space

Mar 8, 2011
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I installed OSX installer on a USB 3.0 flash drive with 16GB of storage capacity. The installer normally takes up about 5,5 GB of space.
Show Info (in Finder), Storage (in About this Mac) and Show Info (in Disk Utility) confirm that there are about 5 GB used, and about 10GB of free space.
However, if I select the flash drive to partition it, the partition window shows that about 10GB are being USED, instead of free, and the largest new partition I can make is therefore 5GB (instead of 10).
I then filled the drive with about 10GB worth of movies and deleted them, so I'm sure that the partition tool is incorrect.
I've been surfing but cannot find a similar situation.
(1: I'm guessing the installer didn't do a "snapshot" of itself on the usb stick... and 2: I obviously didn't remove a bootcamp partition with DU from this thumb drive!)
What can be happening, and what kind of solutions could I be looking at, short of reformatting the drive ?
Any ideas would be much appreciated!

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