With the bigger disks that are no longer supported by MBR there's a constant need to convert to GPT.
We have done this using a live disk, however, it is always important to create system backups. The disk was 6TB, with 2 smaller live partitions. Without converting to GPT we could not take advantage of the entire disk capacity. Assuming that the disk is /dev/sdb (the first 2 partitions are /dev/sdb1 and /dev/sdb2). Below instructions are not for a bootable disk, rather a secondary partition.
Running the above command will automatically prompt you to convert, so you should just press w to save.
Reload the partition:
and you should be done.
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