Early alpha v0.1.0-alpha.2 GPL-3.0-or-later
MjolnirVSS images a running Windows system disk — partition table, EFI, Windows and recovery — and writes that image back onto a blank disk after the original has failed. One executable, run from a folder. No agent, no driver, no service.
Download v0.1.0-alpha.2 View on GitHub
A portable folder for 64 bit Windows, about 2.5 MB. Open source under GPL-3.0-or-later, and in early alpha.
Why it exists
Copying files protects documents. It does not protect the machine: the installed programs, the licences, the drivers, the boot configuration and the hundred settings nobody wrote down. MjolnirVSS copies the disk those live on.
It takes a deliberately portable approach — no installed service, no driver, no scheduled task and no runtime dependency — so the tool you rely on in an emergency is a folder you can carry.
Every partition needed to start Windows is captured together: the GPT table, EFI system partition, Microsoft Reserved, Windows and the recovery partition. If one cannot be read, the backup fails rather than producing a disk that will not boot.
The Volume Shadow Copy Service holds the volume still for the instant it takes to open a snapshot. The image is read from that snapshot, so applications keep running and no reboot is needed.
Every block is read back, decompressed and compared against its BLAKE3 digest before a backup is marked complete. A damaged backup is refused by the restore wizard rather than half written.
No installer, no Windows service, no filter driver, no scheduled task, no registry entries and no runtime to deploy. It runs from a USB stick, and deleting the folder removes every trace of it.
How it works
MjolnirVSS asks VSS for a shadow copy of the system volume and reads the image from that, so the contents are consistent as of one moment. It declares a copy backup, which leaves other backup software's schedules and logs alone.
NTFS is asked which clusters are allocated, and only those are read. Blocks are compressed and hashed, and can be encrypted with Argon2id and AES-256-GCM. A volume that will not report its bitmap is copied whole, and the backup records that it was.
Recovery media is built from the Windows recovery files already on the machine. Booted on the broken computer, the wizard rebuilds the partition layout, writes every partition back and repairs the UEFI boot configuration.
The program
The day you need a backup program is not the day to start reading its manual. The same engine sits behind the window and the command line, so a bug found in one is the bug the other would have had.
The command line runs the same engine
MjolnirVSS.exe inspect # what would be copied
MjolnirVSS.exe backup --destination E:\Backups # copy it
MjolnirVSS.exe verify E:\Backups\PC_2026-09-14_1015
MjolnirVSS.exe recovery-media --iso E:\Recovery.iso # make the rescue media
Every command takes --json and prints one document,
failures included, with the exit code inside it. Eleven exit codes
form a documented contract, so a scheduled task can tell a damaged
backup from a wrong destination.
Tested end to end
End-to-end backup and recovery has been validated across Windows 10, Windows 11 and Windows Server test environments. Each machine was backed up while running, verified, restored onto a blank disk from the recovery wizard, and started without intervention — matching 12 of 12 file hashes and keeping every partition's identifier, offset and size.
| Operating system | Build | Backup size | Result |
|---|---|---|---|
| Windows 10 22H2 | 19045 | 4.75 GB | Restored and booted |
| Windows 11 24H2 | 26100 | 18.28 GB | Restored and booted |
| Windows Server 2019 | 17763 | 4.44 GB | Restored and booted |
| Windows Server 2025 | 26100 | 6.10 GB | Restored and booted |
Windows Server 2022 has not been tested. Sizes are what each backup occupied on disk, not the size of the source volume. Exact test environments are recorded in the testing documentation.
Hardware configurations vary. Test recovery in your own environment before relying on any backup solution, and keep the backup you use today until you have.
Safety
MjolnirVSS sets VSS_BT_COPY. In Microsoft's words a
copy backup doesn't constitute a base backup for further
differential backup operations
, and log files should never be
truncated as a result of a copy backup
. In practice that means
it does not move SQL Server's differential base and does not
truncate Exchange's transaction logs.
Erasing the drive holding the backup halfway through a restore would leave a machine with neither a working system nor anything to recover from. The wizard shows that drive and refuses it, rather than warning and allowing it.
Before anything is written you are shown the target disk's number,
model, serial number, size and current partitions, and you must
type ERASE followed by that serial number exactly.
A single keystroke is never enough.
Every stored block is confirmed present before anything is erased, a disk too small for the layout is refused, and the restore stops if the disk changed size between being checked and being written.
Removing any shadow copy can cost Windows older ones, because the space is released from the oldest end of a single pool. MjolnirVSS deletes only the snapshot it created, by identifier, and says what is at risk before it starts instead of claiming nothing will be lost.
Dynamic disks, Storage Spaces, software RAID, ReFS system volumes, legacy BIOS boot, Windows spread across several disks and untested sector sizes are each refused with an explanation of why.
Design
Nothing about a backup is hidden. The format is specified, the Windows interfaces used are the documented ones, and the whole thing is one static executable with no runtime to deploy.
x86_64-pc-windows-msvc, C runtime linked statically. No redistributable, no .NET, no PythonIVssBackupComponents, declared as VSS_BT_COPY, with the snapshot released by its own identifierverify--json on every command, progress on standard error, eleven documented exit codes, and no prompt that can block a scriptQuestions
From the releases page. It is a zip holding two executables and the documentation; unpack it anywhere and run it. A SHA-256 sum is published beside it. You can also build it yourself from source with the stable Rust toolchain and the Visual Studio C++ build tools, which is four commands.
On the VSS side, yes by design: it declares a copy backup, so it does not move SQL Server's differential base or truncate Exchange's logs. On the maturity side it is early alpha, so run it alongside your existing backup rather than instead of it until you have tested a recovery yourself.
No. There is no installer, no Windows service, no kernel filter driver, no scheduled task and no registry footprint. It uses the shadow copy service that is already part of Windows. Deleting the folder removes it.
Not today. It images the disk Windows boots from. Other disks are detected and named in the plan so you know what is not being captured, but they are not copied.
An unlocked volume is read through its shadow copy, which presents it decrypted, so the backup contains readable copies of your files unless you encrypt the backup itself. A locked volume is refused. A restored disk comes back unencrypted and BitLocker has to be switched on again. No recovery key is ever read, stored or logged.
It can, and so can any program that takes a shadow copy. Windows releases shadow storage from the oldest end of one pool, so removing a new snapshot sometimes removes older ones first. MjolnirVSS deletes only its own snapshot and warns beforehand when there is something to lose. Your files are unaffected.
Yes. A backup is a folder of JSON manifests and compressed blocks, and the format is documented specifically so that an independent reader can be written. Nothing is hidden in a proprietary container.
No. UEFI firmware with a GPT system disk only. A legacy BIOS installation is refused with an explanation rather than half supported.
Why the name?
MjolnirVSS got its name from my fondness for Brothers of Metal, a Swedish metal band I listen to a lot. Their Norse mythology theme led to Mjölnir — Thor's hammer — and the name stuck for a tool built around getting a Windows machine back after a hard hit. The VSS is the Volume Shadow Copy Service, which is the part of Windows that makes copying a running disk possible at all.
The repository holds the source, the on-disk format, the threat model and the full record of what has been tested and how.