How do I format it to primary partition?
You would need to change the partition table scheme from GPT to MBR.
I assume you would loose any existing data on this disk by doing that.
Once having set the partition table scheme to MBR, you would create a new partition and format it.
More importantly, does it make any difference?
Yes it does, for instance from a recovery viewpoint. If the partition table(s) get lost, specialized software is able to rewrite the partition table. As writing more than one partition tables is required, incorrectly determined locations of partitions can cause partition data to be overwritten (worst case: one sector per partition). When rewriting a GPT partition table, write operation takes only place in a reserved area at the beginning and at the end of the disk. Incorrectedly located partitions will not trigger overwritten partition contents.
Recommendation: For data drives use GPT when your environment is compatible to this format.
Explanation of MBR partition table structure - by an example
MBR partition tables are realized by means of one or more linked partition tables located in sector 0 of the drive (to be correct, at LBA=0) and additional partition tables located between the data space occupied by the partitions.
This uncomfortable layout is the result of an extension of the old partition table format which had just one table allowing for four entries. As DOS 3.3 could not handle more than one primary partition the extension was defined.
The following list was created with the backup function of Testdisk, annotations in bold writing. It is a real example of a Windows XP boot disk containg three partitions, a primary and two logical ones.
#1653689348 Disk /dev/sdb - 120 GB / 111 GiB - CHS 14593 255 63
first partition table, at LBA=0
1 : start= 63, size= 4192902, Id=06, * C:-drive, Fat16
2 : start= 4192965, size=230243580, Id=05, E
second partition table, at LBA=4192965
5 : start= 4193028, size=210419307, Id=07, L D:-drive, NTFS
6 : start=214612335, size= 19824210, Id=05, X
third partition table, at LBA=214612335
6 : start=214612398, size= 19824147, Id=07, L drive without label, NTFS
At LBA=0 there is the first partition table. It contains the entries 1 and 2 above. The first entry points to a Fat16-Partition starting at LBA=63.
The second entry at LBA=0 points to the second partition table at LBA=4192965.
The first entry of the second partition table is pointing to my D:-drive.
The second entry of the second partition table is pointing another partition table at LBA=214612335
The first entry of the third partition table is pointing to another NTFS-drive (without a label).
There is no valid entry thereafter, here is, where the chain of partition table ends.
What the above does not show is that the nested partition table structure is sometimes referring to the first extended partition table and sometimes to the partition table before - it's a coding mess.
Comment on a statement by user1686
For data disks larger than 2 TiB, GPT is the only option (unless I guess it's a 4K-native disk?
Generally yes, except for the case of ready-to-use external drives with additional emulating electronics.
Manufacturer has drive with native sectors of 4096 bytes, but the interface acts as if the sector size is 512 bytes.
=> 2 TB limit applies!
Manufacturer has 3TB drive with native sectors of 4096 bytes, but the interface acts as if the sector size is 512 bytes. Manufacturer puts in an additional emulating electronics making the drive appear with a sector size of 4096 bytes.
=> 3 TB can be addresses even with the MBR scheme. Disadvantage: When removing the drive for recovery, the drive geometry changes from 4096 bytes/sector to 512 bytes/sector.
28.5.2022 (rotating) hard drive market
|sector size (internal)
||sector size (external)
||# models currently sold
||GPT required when >2TB
||no, when drive <= 16TB
*1 when adding additional translating electronics, like the one used in "ready-to-use" drives in external housings, GPT is not required
if size of disc / size of sector (external) < 2^32
=> Full 16 TB can be used
=> Only 16 TB can be used from a 18TB drive
Hint: In practice rising hard drive capacities have always disclosed problems in firmware and operating systems. To verify that your machine has no weak link in the chain create one big partition spanning your drive and run h2testw on Windows or f3 (fight flash fraud) on linux.