Don’t go allowing timemachine to make a new backup. If you have an error and it tells you it wants to generate the backup ALL over again (thus losing all your backup history, leaving you without a full backup while it completes and generally being annoying), don’t do it. See if google helps. In my case, this blog would have helped me:
I believe my problem started when my computer decided to sleep in the middle of the backup process. I am unsure if this occurred because of my systems preferences or whether I actually shut the computer lid enough to force it to sleep. Regardless, Time Machine seems to freak out if it gets interrupted during the backup process. When this happens several files and folders are left on your backup volume that deter any subsequent backups.
Solution? Well, check out his site but basically you need to remove the partial files it created so that time machine doesn’t know the last problematic half backup didn’t happen. That allows it to start a fresh backup as if nothing happened, thereby preserving all your historical backup data
[box] Access your backup volume in Finder (it should be named “Backup of your-computer-name…” or something similar) and double-click into the “Backups.backupdb” folder. Next, click into the “your-computer-name” folder. You should see a bunch of folders with dates. These are all the dates corresponding to days you ran a successful backup. At the bottom of the folder listing will be one or two things you need to move to the trash. You may see only one or both of these, but delete the file that starts with a date (it should be the date the backup failure started) and ends in “.inProgress.” Also delete a file named “Latest” if it exists.[/box]
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