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.