PST backups are always a nightmare. If you have a 10GB pst and a user add a single email, you need to backup the entire PST. But there is solutions now for this:
1-) backup pst even if outlook is open
2-) backup pst from local store to central server or location (centralize all your psts backups in one single location)
3-) incremental backups for psts (only backups changes of the psts).
And I read lately Carbonite and Dropbox allow you to backup pst in incrementals. I need to test it in a lab but if this works, you can have your PST in a dropbox and backup is done in an snap
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