EMH backup module

The backup EMH functional in the 31st century.

The EMH backup module

The EMH backup module was a 24th century computer memory storage device carried on Starfleet vessels equipped with the Emergency Medical Hologram program. It was capable of storing data too complex or too large for contemporary systems. As such, it was used to store backup versions of the EMH, on a frequent basis, in case it was lost or damaged.

The module seems to have kept a real-time backup of the EMH, as the Doctor remembered the events of the attack when the module was stolen.

The USS Voyager's backup module was stolen during an attack and recovered centuries later by the Kyrians. This version of The Doctor helped to reshape Kyrian and Vaskan history and served as the Kyrian surgical chancellor for many years before finally taking a small vessel and setting course for the Alpha Quadrant in an attempt to trace the path of Voyager. (VOY: "Living Witness")

The existence of a backup module would seem to contradict "Message in a Bottle", in which Tom Paris and Harry Kim tried and failed to create another version of the Doctor. However, since no date or stardate was given in "Living Witness", the only thing that dates when the module was stolen is Seven of Nine, placing the episode sometime after "The Gift". This means that the module could have been stolen before the events in "Message in a Bottle", or it could have been created afterward and then stolen. The determinative question never directly answered by any episode of the series is whether Voyager launched with this technology on board.

Apocrypha

In the String Theory trilogy of novels, it was revealed that there were several backup EMHs, most of them based on different versions of The Doctor according to when the information was "saved." In the second novel in the trilogy, Fusion, a Nacene who had infiltrated the crew by altering their memories was forced to shut down the Doctor and activate the first backup version (essentially the "original" EMH), as the Doctor knew that she was not who she seemed. Despite his lack of practical experience, the new EMH was still able to confirm that the crew's memories had been altered, as well as explain why Harry Kim and Naomi Wildman had not been affected due to the events of VOY: "Deadlock".