Windows showing up new unknown (Z:) drive

Question

Recently, a new drive with letter Z showing up which cannot be opened to see what's inside. Here is the picture for better understanding.

How to fix it? Sometimes, restarting system fixes temporarily.

Right click menu:

Disk Management view:

It shows the unknown drive without the drive letter and says it is EFI something. It again says it is empty but right click on drive shows it is not empty. Am just guessing with the drive size. So, it could be different drive too. Please look at the following picture for understanding.

P.S. I have upgraded to Windows 10 from Windows 8.1

Answer 1

Courtesy of this thread on the Microsoft Technet Forums, I found the following Powershell solution:

$CimPartInfo = get-partition 
foreach ($CimPart in $CimPartInfo) {
    if ($CimPart.Guid -eq $null) {
        $PartGUID = [regex]::match($CimPart.AccessPaths, 'Volume({[^}]+})').Groups[1].Value
    }
    else {
        $PartGUID = $CimPart.Guid
    }
    "Volume GUID $PartGUID"
    "`tDisk #     :`t$($CimPart.DiskNumber)"
    "`tPartition #:`t$($CimPart.PartitionNumber)"
    "`tDriveLetter:`t$($CimPart.DriveLetter)"
} #foreach CimPart

It’ll print something likes this:

Volume GUID {6c747513-0000-0000-0000-100000000000}
        Disk #     :    0
        Partition #:    1
        DriveLetter:
Volume GUID {6c747513-0000-0000-0000-f01500000000}
        Disk #     :    0
        Partition #:    2
        DriveLetter:    C
Volume GUID
        Disk #     :    0
        Partition #:    0
        DriveLetter:
Volume GUID {6c747513-0000-0000-0000-20023b000000}
        Disk #     :    0
        Partition #:    4
        DriveLetter:    U
Volume GUID {6c747513-0000-0000-0000-20823b000000}
        Disk #     :    0
        Partition #:    3
        DriveLetter:

You can also cross-reference the GUID with the output of mountvol Z: /L to verify.

In your case it is most likely the EFI System Partition. On UEFI installations, it contains the Windows Boot Manager and its BCD (Boot Configuration Data) store – by default, at least.

You can remove the drive letter like this:

mountvol z: /d

You can read more about the mountvol command here.

Answer 2

It could be a recovery, UEFI or other partition which is normally hidden in Windows. To check, press the Windows and enter disk management. Select the Windows Disk Management console and see if that ~256 MB partition is near the beginning -- if so, it's likely for UEFI data.

For more complete information, try a third-party tool such as MiniTool Partition Wizard Free.

If it is just a system partition, you can safely ignore Z: or you can unmount or hide it.