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.
That article you link is wrong. There is no way to specify the retention policy in Windows Server 2012 when backing up to external disks. I don't know why they lie like that... The configurable retention policy feature DOES, exist for the Windows Online Backup feature that you can use in Server 2012. Possibly this is where the confusion arises.
AFAIK backup disk space usage is the same in 2012 as in 2008 R2, which was also supposed to handle retention automatically, but didn't always work. When this occurred in 2008 R2 we would use the Diskshadow utility to manually trim VSS versions from the backup disk, which is what I'd advise you do here.
Use diskshadow to delete oldest VSS copy from Windows Server backup disk:
http://www.bluecompute.co.uk/blogposts/windows-server-backup-manually-delete-snapshots/
EDIT: As Twisty points out, Diskshadow is not needed for this any more, you can use the new WBADMIN DELETE BACKUPS command
Best Answer
Actually, the answer is about Log files, that's what it really comes down to.
When you do a VSS Full Backup you do create a backup of all files, but after that the backup application has the potential to truncate logs on the file system.
In a VSS Copy Backup you persevere ALL application files including log files on the live system. Why is this important?
For incremental backups and differentials, if you constantly are writing over your File System logs certain VSS backups may not know where to pick off.
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