The console comes with an integrated harddrive and also supports external harddrives via USB 3.0. Using an external harddrive to store apps and games requires the drive to be USB 3.0 and storage to be 128GB or bigger.
An external USB 3.0 drive, after being formatted on the console, is normally non-accessible on a PC due to a non-standard Boot Signature being written to the first sector of the drive.
The console supports 500GB, 1TB and 2TB harddrives (state: 09/2019). Internal harddrive uses specific partition GUIDs and partition labels.
|Name / Part Label||Guid||Size (bytes)||Size (GB)|
|System Update 2||24B2197C-9D01-45F9-A8E1-DBBCFA161EB2||7516192768||7|
|Harddrive size||Guid||Partition size (bytes)||Partition size (GB)|
When a USB 3.0 drive is formatted for apps/games usage by the console, it gets rendered unreadable by a PC by writing a special, non-standard boot signature.
By overwriting the custom xbox signature with the regular, expected bytes makes the drive being recognized and mountable on a PC.
The boot signature is located in the first sector of the drive at offset: 0x1FE
|Regular (MBR)||55 AA|
|Xbox One||99 CC|
Note: Even if 55 AA indicates MBR normally, for GPT it is just marking the "Protective MBR", to secure GUID partition against ovewriting by non-GPT aware applications.