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#11
The only way to find out is to search systematically.
1. Run WinDirStat as already advised. That will give you the total number of bytes used by the operating system and user files. There you can also explore all folders for details. It does not, however, not show the shadowstorage
2. for that, open an elevated Command Prompt and type (or paste) vssadmin list shadowstorage. That will show you all the space used by restore points.
The results of 1 and 2 together should add to the 42.3GBs. There are no "mystery files" to look for.