Booting from a GNU/Linux live distro (I used the Ubuntu-Studio 22.04 for the operations described in this article) and using parted we can see the hard disk partition scheme:

Model: ATA kimtigo SSD 512G (scsi)
Disk /dev/sda: 512GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End    Size    File system  Name                          Flags
 1      1049kB  106MB  105MB   fat32        EFI system partition          boot, esp
 2      106MB   123MB  16.8MB               Microsoft reserved partition  msftres
 3      123MB   511GB  511GB   ntfs         Basic data partition          msftdata
 4      511GB   512GB  1258MB  ntfs         Basic data partition          hidden, diag

Here it is the same partition sizes, but in bytes:

Number  Start          End            Size           File system  Name                          Flags
 1      1048576B       105906175B     104857600B     fat32        EFI system partition          boot, esp
 2      105906176B     122683391B     16777216B                   Microsoft reserved partition  msftres
 3      122683392B     510851547135B  510728863744B  ntfs         Basic data partition          msftdata
 4      510851547136B  512109838335B  1258291200B    ntfs         Basic data partition          hidden, diag

The Microsoft reserved partition, according to Wikipedia, should not contain any important data: Microsoft reserves a chunk of disk space using this MSR partition type, to provide an alternative data storage space for such software components which previously may have used hidden sectors on MBR formatted disks.

The 4th partition is flagged as hidden and diag. According to parted documentation hidden means that the partition is hidden to Microsoft operating systems, diag indicates that a partition can be used as a diagnostics / recovery partition.

Once booted into a a GNU/Linux live distro using an USB stick, use the ntfsclone tool (from the ntfs-3g Debian package) to save an image of the NTFS partitions to an USB mounted external hard disk:

ntfsclone --save-image --output /mnt/win11-sda3-ntfs-msftdata.img    /dev/sda3
ntfsclone --save-image --output /mnt/win11-sda4-ntfs-hidden-diag.img /dev/sda4

The small Microsoft reserver partition can be binary dumped into a file:

dd if=/dev/sda2 of=/mnt/win11-sda2-msftres.img bs=1024

The fat32 partitions which contains EFI boot files can be mounted and saved into a zip file:

mkdir /mnt2
mount /dev/sda1 /mnt2
cd /mnt2
zip -r /mnt/win11-sda1-fat32-boot-esp.zip .

In Windows 11 search for the Recovery Drive App and choose to create a Recovery Drive, an USB stick with at least 16 Gb of space is required. All data on the USB stick will be deleted. This drive can be used to boot the PC and to restore Windows 11.

The efficiency of the process is ridiculous: it required more than one hour to copy less than 8 Gb of data!

Boot into Windows 11 and run the Disk Management tool; right click the Windows C: partition and choose Shrink Volume…. Type the space you want to free up.

I wish to create new partitions for the GNU/Linux operating system into the free space I recovered, but the free space is allocated between sda3 and sda4: I prefer to move sda4 just after sda3 and have the free space at the end of the disk.

I booted Linux from the live distro and executed sudo partitionmanager; then it was possible to right-click on the sda4 partition and choose Resize/Move, setting zero for Free space before. The modification must be committed clicking the Apply button.

I opted for creating three partitions: one for the root fileststem, one for the swap and one for the home directories:

For the rootfs partition I reserved about 50 Gb. For the swap I reserved about 16 Gb, checking that it is a little more than the system RAM, in this way I can use the hibernate function (suspend to disk).

WARNING: Hibernation and resume from hibernation is not compatible with Secure Boot.

Booting from some GNU/Linux live distribution may require to disable the secure boot option: enter the BIOS configuration program pressing Del or ESC at bootstrap, then Security ⇒ Secure Boot ⇒ Secure Boot = > Disabled.

Boot ⇒ State After G3 ⇒ S0 State.

• SecureBoot Debian