to unpartition, you right click and select delete iirc.
As far as how long it takes to partition, that is dependent on a lot of things. If it is just a standard HDD with SATA interface, it should not take long at all.
But since you are having dificulties with it, do the following. Download and burn DBAN to a CD. Then boot to the CD with the drive that you want to format installed. When it asks you what to do, type in AutoNuke. That will run a DOD 3 pass on the drive. Then use parted magic to make the drive ExFAT.
If that does not work, then you will have to test the drive to ensure it works.