I'm not Airbus either but my husband is, apparently there is no warning in the FD that it isn't working. There should be a note in the Tech Log, I can't remember how long it's MELable without looking it up. 3 or 10 days?
You can't access it normally during a walk around.
It is possible that the QAR would be able to pick up more data than an older FDR.
I am sure an Airbus pilot will be along soon to answer your q's.