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

To convert using this utility you have to know the physical number of your hard drive

  • Open up command prompt as Administrator
  • Type diskpart and press Enter
  • Type list disk and press Enter
DiskPart List disk
  • Note the disk number you want to convert
  • Navigate to where you have extracted GPTGen
  • Run command gptgen.exe -w \\.\physicaldriveX  where X is Disk number from Diskpart and press Enter
  • 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…