My random thoughts...It sounds like the recorder went from "recording" to no power which would have prevented the save of the data. This has been talked about in a few other threads, usually someone running power but not having batteries in and then losing power. Your scenario is different but the process may have been the same. Unless you are certain the recorder was completely out of "recording mode" and then as a later step you couldn't power down. However even if that were the case, I'd feel pretty certain that somehow you went from the recorder still being in record mode to a no power mode. I am curious if you had thought about removing all the batteries rather than pull the card. Perhaps it wouldnt make a difference, but we are always warned that pulling a card during an operation could damage the card and/or the deck. I think loss of power would be a hair safer.
I have had my Tascam DR100 MKII lock up and not power down, but I always just pulled the batteries, and never lost any data. That said, I think (but who can remember) that I was out of record mode. I always use an external battery on all my recorders always. I always try to have fully charged batteries inside just in case, but I never like to rely on them even if an external battery can be a bit of a hassle at least in a lowpro situation.
I am not sure the best solution, a new card couldn't hurt and according to what youa re saying, an external battery couldn't hurt. There are some pretty small external batteries out there FWIW. Sorry for your loss
It's always a drag when something like this happens.