Tonight, on Galaxy, Witcher 3 GOTY tried to update by about 485mb, it keeps of failing at 99% due to "no disk access". My disk is fine, in fact it updated No Man's Sky on the same disk with no problem.

Could anyone tell me what this update is and if anyone else is having this issue. Thanks.

Hi there.
The new patch added support for Chinese language.
I would look through your installed mods for a possible conflict.

Thanks. But would a mod make an update fail? And as another little bugbear, shouldn't we have an option in games to not install a language and save disk space and download time? Anyway, I will check, as I am planning on another playthough of the game soon!

Hi John.
The most probable reason for an update failure is a "malfunctioning" GOG Galaxy Client. At times the installation fails to create the appropriate registry entries or .dll files fail to decompress properly. That is why a re-install is needed after a thorough uninstall of the GOG Galaxy client, in case you encounter issues with its intended function.
Other than that, there could be an issue with installed mods, since an update would look for updatable files that could be duplicated, in some extend/manner, by mod files, depending on how the mod handles the original files of the game.
Concerning the selected game language, the game uses English as the default one, both for text and voiceovers, unless specified otherwise, while the initial installation of the game takes place, by the user. After the installation is complete, the game language can be changed through the in-game settings. If the voiceover language is switched to any other than English, an "extra" and "beefy" voiceover pack of the specified language will be downloaded. Since all text languages are integrated in the standard installation files, changing text language will not lead to a complimentary download, hence making the Chinese language support update, a mandatory and "lightweight" download for all the copies of the game.
Excuse me for the delayied reply.