For tapers, concensus seem to be that there's little point backing up in the field, most of us go straight home to the computer after the gig anyway.
And flash media being so cheap, most of us just buy a stack of cards if you go on the road for a longer tour.
If you need 100% redundancy for some reason, look at dedicated PSD (Portable Storage Drives), unfortunatey still $120 and upwards (nexto, Sanho, Wolverine etc).
See this old thread from 2008:
http://taperssection.com/index.php?topic=65298.msg1288138#msg1288138Something cool that dropped a lot in price recently is Belkin Media Card Reader for Ipod, street price these days only $20:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250717025256If you have an old Ipod in your drawer, this would probably be the cheapest/smallest solution...