How should I back-up my data before a windows re-install? Omitted files when copying C drive

I wonder how to make sure I backed up all files before whiping out C and re-install Windows (factory reset).

I have taken best of care to copy my documents, photos etc. to an exteal drive. However there is no way I can exclude the possibility that there may be somewhere some files I may need. For example, right after I thought I was done, I found a folder on C where some game had put the saved game files. So I might have lost these if I had not seen them because they were in an unsual location on the drive.

One obvious option seems to be to simply copy the whole system drive (and any other partitions). However, when I do so by ctrl+c and +v (copy and paste), Windows copies far less data volume (in GB) than is on the drive. It seems it systematically omits some files. Maybe these are unimportant system files, but I am not sure.

Hence my question: how should I backup my data before factory reset to make sure I got everything (beyond those things I can easily copy manually)?

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