System Recovery does not work anymore

After the removal of a trojan, it is not possible to perform a system recovery. The data for recovery were stored on an external disk drive, which I connected via USB cable (supplied by the vendor) with the labtop (Dell D830). When trying to perform the system recovery on a certain recovery point, I get the message that the external drive is not recognized, although it blinks during the recovery process. After the recovery, I get the message that the recovery failed because an unnown error occurred.

On the other hand, it is possible to perform the storage for system recovery with this external disk drive.

Info: PC runs under Windows Vista Professional, 32 bit.

All useful hints are appreciated.

Hermann