Then get a portable HDD.
Something like this:
http://www.trademe.co.nz/Photography...n-19542270.htm
(Thats all in NZ$ by the way).
You just have to look for something thats fast, and friendly on batteries.
Most of the portable drives, from what Iv heard, can take 10-15mins to clear a 1gb card, and will often use a whole set of batteries doing.
If you have that sort of time to spare, and the money for several sets of batteries then they can be useful, but I think your still better off either buying a microdrive for your upcoming D20, or lots of CF cards (type II for the upcoming D20)
They are coming down in cost so much now that its not much more expensive that getting a portable HDD, and you don't lose time waiting for a data transfer. You just put a new card in, then worry about getting it all onto your comp when you get home.