Is E: drive really mapped to C:\Users\Admin\Desktop ?
This is all local on the Server that the DB is on, right? Not trying to backup to the local PC of a session connecting to the server via Terminal Services or something like that?
A filemark in the backup device could not be read. There are many reasons why you may encounter a filemark error. Some of them are
media failure may occur on the device where the backup is located.
write failure may occur during the creation of the backup.