Chaum's cash: backup?

(I congratulate Chaum on his ecash trial. this is a milestone.) the recent Chaum press release and information says that cash is "downloaded" to your local computer. here is my question: of course, I'm sure they have some kind of a backup system. would someone care to comment on it? say I download $1000 to my hard drive and it crashes. what to do? if it was like "real" cash, then it really would be lost. if it wasn't like real cash, and the bank was actually tracking it, then they could determine whether it had been "spent" so far or prohibit someone from "cashing it in" like a hold on a check.