There are times, when you as an IT admin have something called a senior-moment and on the brand new server when creating a data store for file server (or virtual machine) you format data partition as MBR (Master Boot Record).
It doesn’t really make any difference unless you want to have:

  • more than 4 primary partition
  • expand partition over 2048GB or 2TB which in my is more important

Ok I know you can add more vmdks of smaller size. You might even combine them in one single dynamic disk but what if you already have data on that storage, lets say 1,9TB and doesn’t have free storage to move data, reformat and move it back?

You need to convert MBR partition to GPT (GUID Partition Table) which introduces some new features, like:

  • Address range expanded from 232 to 264 sectors which enables partition sizes larger than 2TB (assuming 512 sectors)
  • More than 4 partitions per disk. MBR was limited to 4, GPT defaults to 128, but can be increased.
  • Fully supported by UEFI BIOS system. Actually you need to have UEFI BIOS to be able to boot from GPT disk.
  • More at wiki

There are many apps you can use to convert MBR to GPT, but most of them are paid/licensed version since they are meant to be run on server versions of Windows.

There is, however a small command lline utility designed especially for that purpose, it is called GPTGen.

To convert using this utility you have to know the physical number of your harddrive (or vmdk if its virtual)

  1. Open up command prompt as Administrator
  2. Type diskpart and press Enter
  3. Type list disk and press Enter
  4. DiskPart List disk
  5. Note the disk number you want to convert
  6. Navigate to where you have extracted GPTGen
  7. Run command gptgen.exe -w \\.\physicaldriveX  where X is Disk number from Diskpart and press Enter
  8. Now it’s good idea to reboot, then after reboot run check disk on converted disk just in case.

That’s all.

Now you can extend partition to 264 sectors. Assuming you have a legacy disk/scheme with 512bytes sectors, that gives maximum size is 9.4 ZB (9.4 × 1021 bytes) or 8 ZiB (9,444,732,965,739,290,427,392 bytes, coming from 18,446,744,073,709,551,616 (264) sectors × 512 (29) bytes per sector) but in real world Windows cuts this to 256TB per partition which in my opinion is more than enough.

With GPT disk scheme you must have UEFI compatible BIOS or install special MBR-to-GPT boot loader in order to be able boot Windows from GPT disk, but this is another story…