Change destination drive

My original backup was done to an external drive that the computer designated as E:.  Recently I plugged the drive back in and the computer assigned I:.  Acronis won't find the drive.  I went into the change settings and changed the destination to I: It now shows I: as the backup drive, but when I do a backup I get an error message that says it can't find the path to E:.  I updated to the latest versiopn, still no go.  How do I change the destination drive for an existing backup?