I ran into an issue with a user running Windows on a Mac. Windows should have taken up the entire drive (which is 250 GB), but Windows Explorer was reporting about 90 GB.

The really odd thing is that within Disk Management itself, it reported 90 GB on top (same as Windows Explorer) but then 250 GB on the bottom… and no empty space after the partition.

I found the solution here:
I Expanded the Disk in Disk Manager but It Doesn’t Show in Windows Explorer

Launched up cmd.exe as administrator and then ran

and then it was all good.