When I manage my Windows 7 Backup's disk usage through the "Manage space used by this backup" and then "View backups..." options, I see two backup periods.

Windows 7 Backup Disk Periods

That report implies I will free up 1.14 TB if I delete the "12/9/12 to 1/1/13" backup period and leave 84.49 GB of backup data untouched. But how is that possible? My backup target(s) total 517 GB, far more than the second period.

So I am wondering...

  1. Will Windows Backup preserve/promote the file data required for "1/2/2013 to 1/5/2013" to remain a complete backup, therefore resulting in a "1/2/2013 to 1/5/2013" backup period that is 500+ GB in size?
  2. Or will Windows Backup discard everything discovered during the "12/9/2012 to 1/1/2013" backup period, even if the newer backup period needs it, therefore leaving the backup largely incomplete until another backup is performed?

The dialog says "All files backed up during the selected period will be deleted," but I find it surprising that I should have to forfeit ~80% of my backup (a huge date range of incremental data), and then run it again before a disk failure, just to free up some space on the backup target.

  • Is it possible that the backup from 1/2/2013 to 1/5/2013 never completed successfully? I noticed on my system that Windows failed to mark in any way these incomplete backups.
    – alexw
    Commented Sep 4, 2015 at 19:28
  • I suppose that is possible. But like you said for your backups, I haven't seen an error report to suggest the backup failed.
    – jimp
    Commented Sep 7, 2015 at 16:38
  • One of the issues I've faced with Windows Backup is that it doesn't interrupt Windows Auto-update. Numerous times, I've left a backup in progress, only to come back later and find that Windows Update ran and automatically rebooted, interrupting the backup. Only sometimes has it reported a "backup failed" message in that case.
    – alexw
    Commented Sep 7, 2015 at 17:50

1 Answer 1


The granularity for deletion is a backup period, which represents a full backup together with its associated incremental backups.


Each backup period should be self-contained, and unaffected by the (non-)existence of other backups. There are several possible reasons for the size difference:

  • The smaller backup period is only over 3 days, compared to 23 for the larger backup. With a period almost 8 times longer, a larger size is expected.

  • Since this is an incremental backup, an initial full backup is taken and then any changes will be backed up on top of that. If you have made a large number of changes on the drive and/or have a very short time between backups, a larger size is expected. If you change 50 GB of data every day (with a daily backup), that would be 2.65 TB worth of changes that will be backed up over 23 days.

  • A combination of the above

Also, there is a known bug that may result in some files being backed up each cycle, despite being 'unchanged' (if a media service updates the metadata): http://support.microsoft.com/kb/981470

  • I think your quote that defines the backup period is the correct answer. My 3 day range must have failed on the full backup, because there's really no way my data would have compressed that small.
    – jimp
    Commented Sep 7, 2015 at 16:42

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