Time Machine question: backed up 9 gbs, but I was on vacation!

The cleaner's been using your Mac.....

ero87 said:

Hey friends,

Time Machine gives me an error message whenever the memory in my external hard drive gets very low, so i like to keep some extra space in my external hd. Anyway, I just came back from winter break to my iMac, fired it up, and Time Machine had 9 gb of stuff to back up. What?! Where could this have come from, my computer was off, no one touched it!

I'm curious if I have some strange setting on that is causing this erratic behavior. Any advice would be much appreciated 🙂

Click to expand...

This is a commonly misunderstood phenomenon with Time Machine.
When Time Machine says "backing up [x]MB of [y]GB", it doesn;t really mean that it is copying that much info from your computers hard disk to the Time Machine disk. That size also includes the compacting of the time machine backups that it had done in the past.

Time Machine backs up hourly for the the past 24 hours. After 24 hours, it deletes/merges (i'm not sure yet) 23 of those backups and stores just 1 backup for that day. After a month, it again deletes/merges more of the daily backups and creates weekly backups. So the amount of data shown actually includes the amount of data Time Machine has to move around when doing this deleting/merging. I have been testing this precise thing for the past week and this is my finding. I have left my MBP switched on and untouched for the past week and last night i noticed it wanted to backup 47GB of data. My external HDD had only 65GB free. But when it finished the backup, I still had about 64.8GB free. Meaning it just moved the data around and added just about 200MB to the existing backups.

Regarding the deleting/merging confusion, though I haven't been able to come to a precise conclusion, from what i'm seeing, i think it does some kind of smart merging of data but i can't report on that just yet. Will post again once i have conclusive evidence of what goes on here.

Hope this helps.

Or he could have left something like Entourage, Parallels, or Fusion running, without setting Time Machine to ignore the applicable files.

If he had to fire it up after returning from break, there shouldn't have been anything running. The machine was off!