By default, when formatting a partition as ext2/3/4, Linux will reserve around 5% of the space for use by the root user. This is mainly to avoid non-root users filling up the filesystem, as well as helping to reduce fragmentation.
On external drives, this is less important - especially if the content on those drives is pretty static (e.g. a backup drive) - so we can reclaim the reserved space and gain quite a bit of extra storage (over 185GB on a 4TB drive!)...
Firstly find the partition that you want to adjust... don't change the reserved space on any system partitions (including /):
mount | grep /dev/sd
This will print the list of drives and where they are mounted, so you can work out which one to change. Now use:
sudo tune2fs -m [percentage] /dev/[device]
to change it, substituting [percentage] for an integer, and [device] for the device name you found earlier, e.g. the following command will reserve 1% of the /dev/sda1 partition:
sudo tune2fs -m 1 /dev/sda1
P.S. If you want to find out how much space is currently reserved, run:
sudo tune2fs -l /dev/[device]
Divide the number "Reserved block count:" by the number "Block count:" and it will give you the reserved percentage.