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...
DISKPART> list volume
a list of all your lettered volumes should appear
substituting the number
corresponding to your z drive for 'n'
DISKPART> remove letter=z which removes the letter, not the partition
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