We have a Win SBS 2011 Essentials system, SP1, auto installing updates. It's using Windows Server Backup to take daily backups to one of five external USB drives, which we rotate offsite. Windows Server Backup is using the drives as dedicated backup drives.

It appears to be doing this incrementally, and one of the disks has filled up and backups now fail to that disk. Although research suggests Windows Server Backup is supposed to manage its own space and delete old files, it doesn't seem to be in this case. Disturbingly, if only one disk has filled up, that suggests each disk has different backups on it, and if they're incremental, that means we have one single backup that is dependent on all five disks being available and functional.

In disk management, I can see the disk and I can see that it's full. However I can't mount it- if I assign a drive letter to it, on the server or on a different machine, the disk mounts but is inaccessible. The same behaviour occurs when I connect another drive, to see if something's wrong with the disk.

Backups are of specific folders. The backup disks are each the same size as the source disk, so we're not trying to back up more data than the destination can hold. But because (I assume!) they're not system state images, I can't use wbadmin to delete old ones.

I've tried using vssadmin to do it, but get the error "snapshots were found, but they were outside of your allowed context".

In researching, I've found these options;

  • Reformat the backup drive and start again,
  • Get another backup drive each time one fills up,
  • Get a different backup program.

Surely there's a better fourth option? And am I right that there's a somewhat fatal flaw that the incrementals are based on the last time a backup was run, and not on the last time that a backup was run using the presently connected drive?

  • 2
    I'm very happy for you that you caught the "depending on all 5 disks" issue before you really needed the backups. A reminder to all that backups need regular testing to make sure they still work the way you think they do!
    – Grant
    Feb 17, 2014 at 14:57

1 Answer 1


Personally, I would suggest Get a different backup program -- I don't know much about Windows Server Backup on SBS, but I know that dedicated backup programs are specifically designed for the kind of workflow you're describing.

If you want to try to save costs I'd suggest you shelve your current backup drives and buy new ones, then configure a proper backup rotation.

A "proper backup rotation" would look something like this:

  • Every Friday take a full backup of the server. Send that disk off-site on Monday morning.
  • Monday through Thursday take an incremental backup - ideally to separate disks like you're doing now - and send those off-site.

You would need at least 6 disks (Monday through Thursday incremental backups, and two Friday "Full" disks) for this process, and would scratch each disk as you reuse it.
The idea is to always have one "Full" backup set off-site that will allow you to restore to your current state (either last Friday's full backup, or the previous week's full set of disks).

Basically each disk is treated as a (very large) virtual tape in this situation. Depending on how much data you have to back up you may be able to store several weeks of backups on the disks by configuring the backup software appropriately.

As Grant pointed out in his comment, YOU ALSO NEED TO PERFORM RESTORE TESTING when you set up your new backup rotation.

Based on your question I can't tell if you do restore tests regularly, but it sounds like you don't (otherwise you would know for certain if you really need all of your disks for a restore). Backups that have not been restore tested effectively don't exist, and testing them in an actual emergency where you need to recover the system is a Bad Idea because if they don't work you're in a really bad situation.
The usual recommendation is to perform a restore test quarterly, or any time you change the backup set or system configuration.

  • A day later, one answer, two upvotes, looks pretty definite :) Ta. Feb 18, 2014 at 10:07
  • Windows' built-in "Windows Server Backup" is only capable of taking full backups (from a restore perspective). When WSB takes a backup, it performs a shadow copy of the backup storage destination drive, then performs a "full" backup of the source data (source [Note that SBS 2011 is really Server 2008 R2 under the hood]). Bottom line, backup media dependencies don't exist with WSB and every backup drive contains full backups. Sep 23, 2014 at 18:39
  • 1
    Here's another statement from Technet regarding the full/incremental behavior of WSB backups: "You no longer need to manage full and incremental backups. Instead, Windows Server Backup will, by default, create an incremental backup that behaves like a full backup. You can recover any item from a single backup, but the backup will only occupy space needed for an incremental backup." Sep 23, 2014 at 18:49

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