Summary: I'm trying to back up a 1TB drive. The "backup size" is 1.2 TB. Huh?

I'm trying to back up a terabyte drive that's only about 16% full. This is the first time I've run "Backup and Restore (Windows 7)" since upgrading to Windows 10. The software says the F: (external backup) drive has "365.79 GB free of 1.8 TB". It further says: "Backup size: 1.31 TB".

How can the backup size be so huge?

(I'd strongly prefer to keeping using the same backup software, since it has many old, now-deleted files backed up.)


  • OS: Windows 10 Home (upgraded from Windows 7 Home)
  • Internal drive (to be backed up)
    • System Reserved: 100 MB NTFS, 75 MB free
    • C: 930.97 GB NTFS, 779.32 GB free
    • Recovery Partition: 450 MG, 450 MB free
  • External drive (to back up to):
    • F: 1863.01 GB NTFS, 365.79 GB free
  • Backup software: Control Panel > System and Security > Backup and Restore (Windows 7)

"Back up or restore your files" has an error message:

Check backup disk space
The disk that your backups are being saved on doesn't have enough free space.

"Manage Windows Backup disk space" reports the following information for my external drive:

Data file backup: 686.83 GB
System image: 654.95 GB
Other files: 155.44 GB
Free space: 365.79 GB
Total size: 1.82 TB

"Data file backup" says I have four backups before today: 115.01 GB, 175.21 GB, 176.43 GB, 124.41 GB. Today's (failed) backup is listed as 95.78 GB.

(I've already deleted two old backups. In retrospect, this could have been a mistake. Those backups might have been for files that now have no backup, making an incremental backup even bigger. On the other hand, if the backup is bigger than the data to back up, all bets may be off.)

"System image" gives me an option: "Keep only the latest system image and minimize space used by backup (this will free up to 448.61 GB space)" That's tempting, but it wouldn't be enough.

What's going on here?

  • Your data + system images equal the size your asking about. So what is the question exactly? – Ramhound Aug 2 '16 at 19:29
  • Huh, you're right; the "Backup size" is the size of all the existing backups, not something I ever would have guessed. But why, if there's 365.79 GB free on the F: drive, can't I backup 151.65 GB of data (a.k.a. non-free space)? – user10543 Aug 2 '16 at 21:15
  • You have both data and a system image that will be created, the combination of those two, is greater then 365 GB – Ramhound Aug 3 '16 at 2:27

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