Two operating systems, one drive, and the space you cannot see from either
Partitions, hibernation and shared data on a dual boot machine. What each side can read and what to watch.
A dual boot machine divides one drive between two systems, and each of them reports only its own share as the drive. That is the first thing to understand: neither side is lying, and neither can see the whole picture.
Where the space goes
| Partition | Typical size | Visible from Windows |
|---|---|---|
| Windows (C:) | The share you allocated | Yes |
| EFI system partition | 100 to 500 MB | Not as a drive letter |
| Recovery | 500 MB to 1 GB | Not normally |
| The other system | Its own share | Not without extra software |
| Shared data partition | Whatever you chose | Yes, if formatted exFAT or NTFS |
Seeing the whole disk
Get-Disk | Get-Partition |
Select-Object DiskNumber, PartitionNumber, DriveLetter, Type,
@{n='SizeGB';e={[math]::Round($_.Size/1GB,1)}}That lists every partition including the ones with no drive letter, which is the view neither Explorer nor Settings gives you.
Shrinking Windows to make room
Disk Management can shrink the Windows partition, and it frequently refuses to shrink as far as the free space suggests. The cause is immovable files near the end of the volume: the page file, the hibernation file and shadow copies. Turning those off temporarily, shrinking, then turning them back on is the standard sequence.
- Disable hibernation:
powercfg /hibernate off. - Turn off System Restore for the drive temporarily.
- Set the page file to none, and restart.
- Shrink in Disk Management.
- Turn all three back on.
The Fast Startup trap
Windows with Fast Startup enabled does not fully shut down: it hibernates the kernel session. A Linux system that then mounts the Windows partition read write can corrupt it, because Windows believes the filesystem is exactly as it left it. Turning off Fast Startup is not optional on a dual boot machine, and it is the same setting as the hibernation file.
Sharing data
A separate exFAT or NTFS partition both systems can write to is the usual arrangement. exFAT is the safer choice for shared write access, at the cost of no journaling, which is covered in formatting a drive.
Common questions
Why will Windows not shrink the partition further?
Because of immovable files near the end of the volume: the page file, hibernation file and shadow copies. Disabling all three, shrinking, then re-enabling them is the standard sequence.
Why is Fast Startup a problem for dual boot?
Because Windows hibernates rather than shutting down, so the filesystem is left in a state it expects to resume. Another system writing to it can corrupt it. Turn Fast Startup off on any dual boot machine.
How do I see all partitions on Windows?
Get-Disk piped to Get-Partition in PowerShell lists every partition including those without drive letters, which is what Explorer and Settings both hide.
What format should a shared partition use?
exFAT for write access from both systems, accepting that it has no journaling. NTFS works if the other system has reliable write support, which varies.