This sounds like the device that I have been looking for to back up photos while traveling.
I am hoping that I would be able to plug a mini SD card reader into one USB port and a USB hard drive into the other then use the built in web based file manager to copy files from SD to USB HDD using the iPad as a "console".
The important point is that the files would be copied within the TonidoPlug, not through the iPad, over WiFi.
I could also install a 2.5" HDD at a later date of course.
I am already using. DreamPlug to do this but it lacks the web based file Manager, WebDav support etc. Instead I am using X Windows and iSSH, which works but is a bit fiddly. The TonidoPlug seems to be specifically designed to satisfy the need for external storage for tablets.
EDIT: I think perhaps there is only one USB port so I may need to install a 2.5" HDD from the outset.