Just downloaded all 26 parts and 102 GB of the latest patch (1st Dec 2023) to BG3, and when I run the installer it tells me there's not enough room to update the game. There's 1.2 TB free on the disk, the installer wants 1574969 MB, or 1.57 TB.

The game it's patching is 120 GB, it's asking for more than ten times that to patch it. I'm going to have to download the game *again* and reinstall it from scratch. GoG's patch 5 installer is utterly hopeless. It's only a couple of GB smaller than downloading a full install, and it needs ten times as much disk space to run.

Their patcher doesn't seem to be able to scale well to large games.

This forum is primarily for Baldur's Gate 1 and Baldur's Gate 2. If you have a problem with the GOG patch for Baldur's Gate 3, the best place to report it would be with someone who can actually fix the bad patcher. This is a community forum, and it is unlikely any GOG staff will even notice your problem report here.

Try posting in the BG3 forum as there have been a couple of posts much like yours. I don't think there's anything to be done (I don't own the game) but you might get heard there. Good luck.

https://www.gog.com/forum/baldurs_gate_3

1 you are downoading off line installer and not using galaxy... i point this out so other people know what you have when they try to help

2 the current build is 4.1.1.4061076 and that just now went live so check this is in fact the one you downloaded and not the older 4.1.1.3956130 [hf3] or 4.1.1.3911062 because if you don't have that [40] in the build number then its worthless now

the error you describe happens on modern Windows systems and translates to your C drive is full or not allowing access... there are a few ways to get it but i'm going to assume you tried to install the patch to a external caddy of some sort... if so then that is our error and its a basic Windows problem that has nothing to do with Gog or BG3

the fix is to clear up space on your c or d [if you have another internal drive] and install the game there

p.s, yes this is the wrong forum as well but good luck

Clear up 1.6TB for the patch to install?

The patch itself is 100GB. I doubt it needs 16 times the space to unpack.
I disagree this is a windows issue