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) :
Start typing...
C:\Windows\system32> diskpart
DISKPART> list volume a list of all your lettered volumes should appear
DISKPART> select volume n substituting the number
corresponding to your z drive for 'n'
DISKPART> remove letter=z which removes the letter, not the partition
DISKPART> exit
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
DISKPART> assign letter=z
