Which full backup will be differential one based on? I wanted to perform a differential backup of a DB in SQL Server 2008.
I currently have a .bak file with a full backup and I want to add this new one in it.

Will be the differential backup based on the existing one contained in the .bak file? Or will it be based on another full backup?
Will be based off your most recent full backup, a differential backup is simply a backup of changed pages since your last full backup. Hope that helps.