Backup of a SAP DB database

Hello     
 
I want to make a backup of my existing database called MIM. I want to
make this to put this database on another computer.
 
I follow the menu "Instance --> Backup --> Complete Data " in SAP DB
Database Manager to create a new medium with the following values: 
 
Backup type: Complete Data
Name: mimSapDB
Location: E:
Device Type: File
 
--> Button "Next Step" 
 
and after that the Button "Start"
 
But I get the following error:
Database: MIM
Error Code: -24988
Description: sql error [backup_save "mimSapDB" DATA RECOVERY]; -903,
Message not available, access denied[5]
 
What does this mean?
 
Why does this way of making a backup doesn't work? 
 
Could someone give me a hint how I can make it on another way?
 
Thanks
 
Bye
 
Ivonne Schallnus
 
 
PS: Please apologize my bad english.