Can't resize external hard drive partition I remember very clearly back when I formatted the drive, that I chose everything that St. Steve prescribed me to, GPT table, HFS+ partitions, even though other platforms can't write to HFS+.

Now, surprise surprise, a year later when I have to partition the drive, Disk Utility says "This partition can't be modified", and diskutil(8) says "Volume format does not support resizing". 

It's not encrypted, there shouldn't be anything in the way.

/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk3
   1:                        EFI EFI                     314.6 MB   disk3s1
   2:                  Apple_HFS Nemo                    3.0 TB     disk3s2


Unable to Resize Partitions talks in depth about resizing corestorage, which is irrelevant here.
(Originally posted as a comment)

Interesting point, I've just erased all partitions and started over, and I noticed that they couldn't be resized after being ejected. However, if I plug the disk back in without unmounting the partitions, they seem resizable. 

So it might be just because I had the healthy habit under Linux to unmount my partitions before fiddling with them.