After a catastrophic HD crash I bought a small 250GB external hard-drive an started to use Time machine. Since my Mac was freshly formatted, I tend to keep it tidy and clean and keep good track of what comes in. I noticed my time machine backing up 15 GB of data which allegedly would have been added within two days. I could absolutely not trace this on my computer nor when I compared current folder contents with the most recent backup. Downloads, Apps., System are all excluded from my time-machine and couldn't represent such a dramatic data increase on my separate HD. Any clue anybody?