Time Machine Not Backing Up

Anyone have an idea why my time machine backup continually runs about 9.2Gb smaller than the source drive? I've found that at least two of my IDVD projects for some reason are not backing up which accounts for roughly 7.56Gb of the missing 9.2. In addition my photos folder is roughly .06Gb smaller than the source folder and my Library folder is .6Gb smaller.

I'm running a 2006 Mac Pro with a operating drive size of 750Gb and an OWC firewire 800 external 750Gb for the backup drive. The back up drive was previously erased prior to running Time Machine.

Thanks for the help.

Because it doesn't back up "everything" on the source drive. Many folders are excluded by default, others you may have added yourself. The size difference in what you are seeing in your photos and projects may simply be some sort of cache, hidden files like the resource forks, and so on that are relatively extraneous and not a critical part of backup needs. Try restoring one of your iDVD projects (after manually backing up the original) and see for yourself if anything is really missing.

You were right! Although the backup IDVD file size is significantly smaller, everything seems to be working fine when I open the backed up file. What a relief!

Thanks for the help.

Were you looking at just the backups for a given hour? I know that time machine will only back up changed files and use hardlinks to non-changed files in the time machine so it's feasible that if it was last changed on day X at noon and that's when it was backed up, then all that would show in later backups would actually be links to the last changed backup instead of an actual new backup.

This explains it a bit better then I have...
http://en.wikipedia.org/wiki/Time_Machine_(Apple_software)

you can pretty much consider what time machine is doing as an incremental backup from the last full backup.