I have the drive plugged in to my router and while it works with 4TB drives it sometimes gets upset when it tries to scan the drive for errors, which is to do with the router not having enough memory to allocate space for the file table. A simple fix to this is to partition the drive. I wanted to resize the single partition and then create a second partition but Partition Manager had other ideas. It tried to resize the partition but decided it couldn't but for some reason decided it would still format the new partition, which it didn't make, and so reformatted the entire drive instead!
It's all fixed now, but there is a lesson here...
If you plan on doing something just don't bother and go back to bed instead :D