VBoxManage createhd --filename hard_drive --size 6144 --format VDI --variant Split2G
Progress state: VBOX_E_FILE_ERROR
VBoxManage: error: Failed to create hard disk
VBoxManage: error: Could not create the medium storage unit 'hard_drive.vdi' (VERR_VD_INVALID_TYPE)
VBoxManage: error: Details: code VBOX_E_FILE_ERROR (0x80bb0004), component Medium, interface IMedium
VBoxManage: error: Context: "int handleCreateHardDisk(HandlerArg*)" at line 423 of file VBoxManageDisk.cpp
This is actually a SOLUTION and not a problem, as I spent a few hours this morning wondering why this was failing and could not google anything that gave me an answer.
You can't use the Split2G variant with a .VDI format. You need to use another (I chose .VHD). The error message is rather uninformative and didn't point to the answer. I hope this helps someone else to a quick resolution.