BackPacker, it sounds as if the switch is in approximately the same place, 30cms or so up from the base of the panel and immediately above the ECU power check button - but in that case I would have expected the HSI switch to the left of it to be hit rather than the ECU Swap. I'm amazed that anyone managed to hit it. From what I remember being told, the problem with the DA42 was that there is (or was - I'm not sure what the modification is) NO back-up electrical power available unlike the DA40 which has approx 30 mins available.
All this just serves to demonstrate the importance of actually reading the POH and not just making assumptions that because you have flown something similar (2 wings, 2 engines, retractable gear etc) that everything will be exactly the same. I was very fortunate in that my conversion to the DA40 was done by the CFI who laid great stress on the importance of reading the POH, especially before a new aircraft was flown.
virtually no single-engine POH (including the DA-40) mentions anything about an external power start,
The POH for the C152 does.
