i do a backup of my stuff manually each month via an ext. HD.
right now, i was backing up my AppData folder (so much is in there!).
being anal, i compared the size of my laptop's internal HD compared to the ext drive when it was done.
internal: 5+ BG
ext. 2+ GB
so i compared the size of each subfolder to see if they match up and, yeah.
conclusion: windows is hiding something from me.
p.s. yes, i have "show hidden files and folders" ticked.