You're correct that all the 7 series nuvis need the power switch to go to the lock position and won't enter preboot without that step. I haven't had a particular problem with flash not completing like that but it's a handy hint to know to move back it to the central position during the flash, thanks for posting that SF.

Here is definitely the right place, i've added a note into initial post pointing to these 7xx specific posts because they are quite different and difficult for preboot attainment.