WARNING! Backup your data before proceeding!

For me this process didn’t destroy the data. I completely expected it to, but it didn’t. Make sure you backup in case you’re not as fortunate.

Move into Single Users Mode
sudo init 1

Unmount partition in question
sudo umount /mnt/disk1

Launch PartEd pointing at the disk in question
sudo parted /dev/sdb<br />
Make the Partition Table GPT
mklabel gpt<br />
Accept the warning and quit PartEd
Warning: The existing disk label on /dev/sdb will be destroyed and all data on this disk will be lost. Do you want to continue?<br /> Yes/No? yes<br /> (parted)<br /> quit

Check the disk (resize2fs required this before it would resize the partition)
sudo e2fsck -f /dev/sdb1

Resize the partition to fill the total available disk space
sudo resize2fs /dev/sdb1

Mount the disk again
sudo mount /mnt/disk1

And hey presto!