MYSTERIOUS "SYSTEM (Z:)" SUDDENLY APPEARED

Second reply:

This is a "permanent" fix!   So far,  It has only been a few days, but my Z: drive has "gone away" and hasn't come back!  Yet.  I know these are a lot of steps, but they work.

I got this from Microsoft;  the Z: drive/partition occurs when a peripheral hooked to your PC tries to do something, such as a printer checking its status or attaching an external drive.  (It seems to me there is a fault in the OS!)  The system creates this drive to accomplish that task but for some reason it does not “hide” that drive.

This is what I was told to do.

First you must open the command prompt window in an “elevated” mode:

How To Open an Elevated Command Prompt in Windows 10 or Windows 8

If you're using a keyboard with Windows 10 or Windows 8, you can open an elevated Command Prompt quickly from the Power User Menu. Just press the WINDOWS and X keys together and then click on Command Prompt (Admin). Click Yes on any User Account Control messages that might appear.

OR:

The following process only works for Windows 10 and Windows 8 which is unfortunate, because it's super-simple and works for elevating other programs as well, not just Command Prompt.

Open Task Manager. The quickest way, assuming you're using a keyboard, is via CTRL+SHIFT+ESC . 

Once Task Manager is open, tap or click the File menu option, followed by Run new task.

Note: Don't see the File menu? You may first have to click or tap on the More details arrow at the bottom of the Task Manager window to show a more advanced view of the program, including the File menu.
 

In the Create New Task window you see now, type the following in the Open text field:

cmd

...but don't do anything else just yet!
 

Check the Create this task with administrative privileges. box.

Note: Don't see this box? That means that your Windows account is a standard account, not an administrator account. Your account must have administrator privileges to be able to open an elevated Command Prompt this way

Now click or press on OK. Follow any User Account Control requirements that might appear next.
 

Either way:

An elevated Command Prompt window will now appear, allowing unrestricted access to executing commands.

Once you have opened the elevated command prompt, at the C:\xxxxxx\zzzz line type in:

diskpart      press enter.

You should see:     DISKPART>

Now type:    list volume      press enter.

You should see all the drives on your PC that you should have access to.   Including the drive Z:   Make a note of the volume number listed under the ### column for drive Z:.  I am going to use 4 as my drive Z:  , just as an example, yours may be different.

Now type:   select volume 4       press enter.

You will see:   Volume 4 is the selected volume.

Now type:     remove letter Z      press enter.

You will see:    diskpart successfully assigned the drive letter or mount point.

You can close the black prompt window,  if everything went okay, check for your drives and Z; should no longer be seen.