Hey guys I'm new to using GOG Galaxy. I installed the game initially on my main drive that is an SSD. However, due to it getting filled up I moved Witcher 3 to my secondary HDD. I downloaded the 1.03 patch and tried running it by finding the folder but it says that "Game installation not found, please reinstall". When I moved the Galaxy client to my HDD it began to download a ~600MB patch so I left and came back to notice that it failed to update the game. I thought restarting my computer may help but now I can't even launch GOG Galaxy. Now I'm stuck, I can still start Witcher 3 and play it on patch 1.02. I'd be grateful for any kind of help thanks!

Galaxy client is not supposed to be moved just like that. There are settings in the registry that are pointing to the old location. You should just uninstall and install again if you want to change location for some reason.

You can change location for GOG Games in Galaxy settings.

After installing Galaxy again, maybe verify/repair for Witcher, from Galaxy could also help.

So, I ended up moving W3 back onto my SSD and the patch installed without a hitch and I ran CCleaner to remove any missing registry. . . thing is that even after that having GOG Galaxy installed on my HDD it said essential files were missing. Restarting the computer fixes this as GOG Galaxy will automatically kick in when the computer restarts. Now it seems like everything works off my HDD (moved W3 back onto my secondary now). Thanks for the reply!