Be wary, it seems RPG game, and players have figured out what seems to be the cause.
The issue was flagged on developer CD Projekt Red's forums, where a number of s stated they'd lost progress due to files becoming corrupted while playing through Steam. Over the course of the posts, a common factor emerges that the saves that are being lost are all over 8mb, 8192kb being the exact magic number that can make your play-through unreadable. From there, discussion moves onto what's actually making this happen, and it seems to be linked to crafting.
Essentially, crafting weapons and items causes a bloat in your save file, and as of now, the open-world game has a ceiling on just how much of that it can handle. This is the working theory, and SigilFey, a forum mod, explains that what's likely happening is to do with internal item ID numbers. "If I am rapidly adding/removing items from my inventory, I may inadvertently increase that ID number to something that the engine no longer recognizes," his comment reads. "Or a number that conflicts with some other function of the inventory. Or save system."
At time of writing, CD Projekt has put up a center post on save corruption, tacitly agreeing with the above, and asking players to "use an older save file to continue playing and try to keep a lower amount of items and crafting materials". The 8mb limit on saves might be increased in an patch, but any corrupted files are unrecoverable.
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