How do I grow an HFS partition on an external USB disk?

Question

I have a USB disk, MBR partitioned, with one ext4 and one HFS+ partition. There is 250GB of unpartitioned space following the HFS+ partition, into which I'd like to grow for Time Machine use.
bash-3.2# diskutil list
-snip-
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *1.0 TB     disk1
   1:                      Linux Backup                  549.1 GB   disk1s1
   2:                  Apple_HFS Mackup                  233.1 GB   disk1s2
-snip-
diskutil refuses to touch the partition, probably because it's MBR ("FDisk") partitioned:
bash-3.2# diskutil resizeVolume disk1s2 limits
Error obtaining resizing information (is this a Mac OS 9 compatible "wrapped" HFS volume?)
How am I going to fix this? Assuming I am unable to copy the contents of my HFS partition to another temporary place.
Thanks

Answer 1

I replaced my MBR with a GPT partition table using gdisk on Linux.
Now diskutil correctly identifies it as GPT:
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:       Microsoft Basic Data Backup                  549.1 GB   disk2s1
   2:                  Apple_HFS Mackup                  233.1 GB   disk2s2
however it still refuses to touch it
bash-3.2# diskutil resizeVolume disk2s2 limits
Error obtaining resizing information (is this a Mac OS 9 compatible "wrapped" HFS volume?)