Coexistence of internal and external SATA HDs?

[grufti]

I used a DUO2 (BH 3.01F) with a 2TB WD HD internal. This HD is almost full. Now I installed via eSATA a 3TB WD HD external. The external HD is initialized and mounted by the device manager.
HARD DRIVE ATA WDC WD30ERFX-68E
Size 3000.591 GB Mount: /media/hdd Device: /dev/sda1
HARD DRIVE ATA WDC WD20EURS-63S
Size 2000.398 GB Mount: /media/hdd Device: /dev/sdb1
I can record and delete files on the external HD. But my problen is: I can see the internal HD only, if the external HD is OFF. It seems impossible to use both HDs at the same time. This is very uncomfortable, when I want to move files from the internal to the external HD. What goes wrong?

 

[grufti]

Update:
Choosing another mounting point did not help. When I tried to copy records to PC and back to the new HD, the receiver made a shut down after some ( > 20 ) hours (Maybe a record tried to start and stopped). After this the external eSata HD was not found by the receiver anymore.
I have bought now a 2 TB Toshiba Canvio USB HD. Same problem. Initialized without error, mounted by device manager as usb and in the second try as video. The external HD can not be seen in the record list and calling the Geräteliste (uppermost position of record list) gives the message "fehlerhafter Ort" with an empty record list. After this I have to use the device manager again to mount the internal HD.