You can't. The ipad doesn't support the transfer of files off of it.
Now if you have a way to plug the drive into the internet somehow you can use Dropbox or some other data transfer programs to upload the photos to dropbox or like service (although I wouldn't do it in 4 gig batches that'll take hours)
There are some wifi harddrives out there that have their own wifi transmitter, the Seagate GoFlex is one, but its reviews are mixed.
honestly, rather than frankensteining something your most user friendly way to do it is with a regular comptuer and card reader. A macAir, card reader and usb port can do what you want in minutes while the iPad will be at the mercy of wifi transfer feed and the use of some other device to store the photos.
With the iPad's Camera Connection Kit, you can plug the SD card into CCK & view thumbnail pics on the iPad without importing the pics onto the iPad. If you see a particular good pic, just import it for full size viewing.
Seems like your most inexpensive solution is to buy some additional and/or larger SD cards and transfer the pics after you return home.
I think maybe the best way to get photos (4 GB batches from a single card) is something called
the Sanho Hyperdrive which I saw advertized on the internet. It is a portable drive that accepts many types of
photo-cards. For capicity it comes in more than one model, starting at 250 GB, I recall, which is fine for my travel needs. It ain't cheap at about $300 but this may be the only way to solve the problem when I am traveling, apart from schlepping around my heavy old MacBook (older model) or buying a MacAir book. I really want the Ipad for traveling now, since I can load photos onto it, edit them, and use Pages and Keynote (at least in theory).