Here are a couple of helpful tips (and notes so I don't have to look it up again!):
1) Use MBR instead of GUID Partition Table. More support for MBR, it is old, but it's restrictions (only 4 primary partitions; max 2TB partition) don't usually interfere with most people's operations.
2) If you get an error, make sure to click on 'details' and then expand the arrows to find the cause of the errors. On my first attempt at resizing I didn't leave enough room for the original partition. To fix, allocate less space for the new partition.
3) The other error I encountered was something about a 'NTFS Journal file is unclean'. I think it was specifically ERROR(95). To fix, use
ntfsfix /dev/[your_partition]. For me, it was
/dev/sda1that I was trying to resize, so
[your_partition]was replaced with
Remember to sit tight, resizing (especially on a TB drive) is no laughing matter! Go for a bike ride, see a movie, and let it do its thing.