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.

 gdisk /dev/sdb

Running the above command will automatically prompt you to convert, so you should just press w to save.

Reload the partition:

partprobe /dev/sdb

and you should be done.

ALT IT Consulting is a full service IT support company, with offices across Canada and United States. If you need assistance with specialized IT projects, feel free to contact our IT support team.