I got a message: Installation failed: Disc access problem
This was after GOG update.

What' did I do wrong?
Played OK until I went to work

???

What do you mean?
I've been running Witcher 3 since it launched with no problems.

The disk may be full or write-protected

try disable antivirius when galaxy download and install patch i have avast and disable help me you can turn on antivirus after download and install patch

Was having the same issue. Followed the above advice of running in Admin and disabling Avast. Can confirm it fixed the problem (not sure which one as I did both at the same time).

I think it likely that it's a anti-virus issue.The update just installed while I was playing,and the game shut down & Windows Defender notified me it needed to run a scan. This did NOT happen with patch 1.10 or hearts of stone.We'll see what happens when defended finishes the quick scan.

Although it could be an issue with your antivirus, I doubt it. "Disk Access" issues are more normally due to rights management. Just run as administrator to see if that works.

I

How do I set it up to run as admin??

Tried disabling virus protection...didn't work.

Right click it and select "Run as Administrator".