The way I read the manual (won't know for sure till I get the Drive next week) it should save the programme from the beginning if the TV has been tuned. I suspect hitting record now makes it a programme to be saved in it's memory somewhere rather than it rewriting over the previous 90 minutes.
I hope that made sense. If it's temporarily saving the information for the previous 90minutes, then it doesn't seem that difficult a concept that by hitting record you're telling it to now permanently save what it's been temporarily saving.
But that's just my uninformed .02