Accessing Volume Shadow Copy (VSS) Snapshots from powershell

Question

I am attempting to create and access a Volume Shadow Copy snapshot using the Windows Power Shell in Windows 7. I found that I can create snapshots using the following via a previous superuser question:

(Get-WmiObject -list win32_shadowcopy).create("C:\","ClientAccessible")

I am unable to find any documentation indicating that it is possible to set a shadow copy to "Exposed" so that it can be mapped to a drive letter using WMI. An article linked in the same question shows a hack using a junction to access the snapshot.

When I try to access the symlink, I get the following:

PS C:\Windows\system32> ls C:\shadowcopy
Get-ChildItem : The parameter is incorrect.

At line:1 char:3
+ ls <<<<  C:\shadowcopy
    + CategoryInfo          : ReadError: (C:\shadowcopy:String) [Get-ChildItem], IOException
    + FullyQualifiedErrorId : DirIOError,Microsoft.PowerShell.Commands.GetChildItemCommand

Attempting to access the snapshot directly gives the following:

PS C:\Windows\system32> ls '\\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy14'
Get-ChildItem : Paths that begin with \\?\GlobalRoot are internal to the kernel and should not be opened by managed applications.
At line:1 char:3
+ ls <<<<  '\\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy14'
    + CategoryInfo          : NotSpecified: (:) [Get-ChildItem], ArgumentException
    + FullyQualifiedErrorId : System.ArgumentException,Microsoft.PowerShell.Commands.GetChildItemCommand

How can I access a VSS snapshot from a powershell script?

Answer 1

How did you create the symlink? As outlined in that article, you have to specify the device path with a trailing backslash:

$s1 = (Get-WmiObject -List Win32_ShadowCopy).Create("C:\", "ClientAccessible")
$s2 = Get-WmiObject Win32_ShadowCopy | Where-Object { $_.ID -eq $s1.ShadowID }

$d  = $s2.DeviceObject + "\"   # <-- this here

cmd /c mklink /d C:\shadowcopy "$d"

After this, I was able to access the shadow copy mounted to C:\shadowcopy just fine.

To unmount the shadow copy call $s2.Delete(), as @KeyszerS pointed out in the comments.

Answer 2

So with the information provided here and the official microsoft documentation here https://docs.microsoft.com/en-us/previous-versions/windows/desktop/vsswmi/create-method-in-class-win32-shadowcopy I created a couple powershell functions/cmdlets that solve this problem. Random note, as of powershell 5 new-item has the itemType of symbolicLink but when I tried to make one with the target being the shadow snapshot it fails saying the path doesn't exist, so the mklink tool is still the way to go.

function New-ShadowLink {
[CmdletBinding()]
param (
    $linkPath="$($ENV:SystemDrive)\ShadowCopy"
)

begin {
    Write-Verbose "Creating a snapshot of $($ENV:SystemDrive)\"
    $class=[WMICLASS]"root\cimv2:win32_shadowcopy";
    $result = $class.create("$ENV:SystemDrive\", "ClientAccessible");
    Write-Verbose "Getting the full target path for a symlink to the shadow snapshot"
    $shadow = Get-CimInstance -ClassName Win32_ShadowCopy | Where-Object ID -eq $result.ShadowID
    $target = "$($shadow.DeviceObject)\";
}

process {
    Write-Verbose "Creating SymLink to shadowcopy at $linkPath"
    Invoke-Expression -Command "cmd /c mklink /d '$linkPath' '$target'";
}

end {
    Write-Verbose "Created link to shadowcopy snapshot of $($ENV:SystemDrive)\ at $linkPath";
    Write-Verbose "Returning shadowcopy snapshot object"
    return $shadow;
}

}

function Remove-ShadowLink {
[CmdletBinding()]
param (
    $shadow,
    $linkPath="$($ENV:SystemDrive)\ShadowCopy"
)

begin {
    Write-verbose "Removing shadow copy link at $linkPath"
}

process {
    Write-Verbose "Deleting the shadowcopy snapshot"
    $shadow.Delete();
    Write-Verbose "Deleting the now empty folder"
    Try {
        Remove-Item -Force -Recurse $linkPath -ErrorAction Stop;
    }
    catch {
        Invoke-Expression -Command "cmd /c rmdir /S /Q '$linkPath'";
    }
}

end {
    Write-Verbose "Shadow link and snapshot have been removed";
    return;
}

}

These could be utilized by copy pasting both functions then running them like this

$shadow = New-ShadowLink -Verbose;
ls C:\ShadowCopy # would show snapshot version of c drive
Remove-ShadowLink -shadow $shadow -Verbose;
ls C:\ShadowCopy # will give error as it doesn't exist

$s = New-ShadowLink -verbose
VERBOSE: Creating a snapshot of C:\
VERBOSE: Getting the full target path for a symlink to the shadow snapshot
VERBOSE: Creating SymLink to shadowcopy at C:\ShadowCopy
VERBOSE: Created link to shadowcopy snapshot of C:\ at C:\ShadowCopy
VERBOSE: Returning shadowcopy snapshot object
PS C:\> ls C:\ShadowCopy
    Directory: C:\ShadowCopy
#ommitted my C drive listing, but it would be here
PS C:\> Remove-ShadowLink -shadow $s -Verbose
VERBOSE: Removing shadow copy link at C:\ShadowCopy
VERBOSE: Deleting the shadowcopy snapshot
VERBOSE: Deleting the now empty folder
VERBOSE: Shadow link and snapshot have been removed
PS C:\> ls C:\ShadowCopy
ls : Cannot find path 'C:\ShadowCopy' because it does not exist.

Answer 3

This is because the Invoke-Expression in the New-ShadowLink returns a string. If you modify the line to:

Invoke-Expression -Command "cmd /c mklink /d '$linkPath' '$target'" | Out-Null

it will fix the problem.