Your backup server is likely using local disks or some form of network attached storage to retain your client's backups. Given this, it's entirely possible that you experience disk errors or dreaded Windows SMB "Delayed write failures" when writing data to Linux/UNIX based NAS devices (if oplocks are not correctly configured in Windows and Linux for this scenario). So the question is, how will your customers be able to restore data if you've lost some?
1. We strongly encourage you to configure client-side local backups (a local vault on say a USB drive attached to the client's computer) in addition to your remote backup vault(s) on your server(s). In this way, synchronization between the vaults can occur in the event that one of the vaults loses data. In addition, the client will try to restore from the local vault before trying your remote vault(s) so restoration speeds will typically be faster than remote only restores.
2. Another possibility is that you configure your backup clients to backup data to two or more remote servers. [steps for adding multiple offsite vaults] In this way, synchronization between the vaults can occur in the event that one of the vaults loses data. (Please contact our technical support group for information on how to configure this for all your installations; to configure this on a per client basis open the corresponding client using the /host option).
3. A third possibility is that you do your own server-side backups (or remote syncs) of your client's data.
All of these strategies involve at least two copies of all your client's backups existing somewhere. That is the only way you can recover their data if you lose some or all of it in a single backup vault.
The client software will automatically synchronize between backup vaults at the end of each backup (or one can run a synchronization from the client using a button on the Backup->Settings tab or via the -sync (or -synclocal) command line flags to the client side backup engine. An additional synchronization option is available on the Backup->Settings tab of client (labeled "Rigorous sync") or via the INI setting rigoroussync; this option if selected will slow down the synchronization process, particularly if there are hundreds of thousands or millions of blocks, but will look in each storage vault for the presence of each block and try and recover missing blocks from a different storage vault for that client (which is only possible if there are multiple vaults).
So, if you suspect loss of block files from a client's storage vault, and there exists more then one vault for the client, you can turn on rigorous syncs for the client and run a manual sync or just wait for the conclusion of its next backup.