Windows 10: Unknown System (Z:) Partition or Disk.

Some bit of indigestion during an update has left one of your system reserved partitions on the hard disk hanging out. I don't think anybody's come up with a set-in-stone solution yet : the problem appears to be random, sometimes disappearing after a few
days. (you can Search these forums for other similar issues)

But, seeing as it's been a few months, if you're feeling adventurous, you could run DiskPart and try and remove the letter.

Open up a Command Prompt (Administrator) : <Win+X> then select from the Powerusers Menu...

Start typing...

a list of all your lettered volumes should appear

substituting the number
corresponding to your z drive for 'n'

If that works, restart and see if the change is persistent.

If horrible things start happening, reverse the process... list and
select as before then