This is a widespread problem this is what i did:
I tried the cmd aproach (subst G: C:\) when trying to install an Adobe update and it worked for me but then I needed to install something else and the problem would appear again.
Checked the registry and I didn't find any obvious G drives so I didn't modify at all.
Then I noticed 2 hard drives in my office and as I have read about this before I connected them both and tadaaa!!! the installation worked.
I think this has to be with some kind of error when connecting and disconnecting external drives. Hope this helps.
Did this solve your problem?
Sorry this didn't help.