if you have SBS 2011 and you or your client installed SharePoint sp1
Backups will fail
Backup unsuccessful. A volume shadow copy service operation failed. Unknown error (0x800423f0)
see Backup has started to fail and will not work SBS 2011:
RESOLUTION: Run the psconfig command
You need to fix SharePoint before your backup will work.
1. Open an Administrative command prompt.
2. Change directory to C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\BIN
3. Run PSConfig.exe -cmd upgrade -inplace b2b -force -cmd applicationcontent -install -cmd installfeatures