Good morning Jan,
There can be a number of possible issues causing the 3 MB/min throughput, which I agree is totally unacceptable for local file backups. First thing I'd suggest to test is to see if it's the tape hardware or SCSI bus causing the bottleneck. Create a Backup-to-Disk folder, and target a backup job to that destination, and see if the throughput increases. If so, then you'll need to check the SCSI controller and bus to the tape hardware(firmware, drivers, cabling etc) .
The Administrators Guide in .PDF format is installed on the server by default, and can be accessed under the Help menu within Backup Exec.
Regards,
Greg