The N810 has a micro USB socket, so any USB data lead with a Micro USB connector should work fine.
Some questions...

1. Go into Settings, Control Panel Memory, Virtual tab, and check to see if you have "Extend virtual Memory" ticked.

2. Do you have a mini-SD card (or a microSD in a mini-SD adaptor) in the slot (known as the external slot) under the stand?

If the answer to the first question is yes, then you will not see the 2GB which is fitted internally to the N810, as it is in use by the tablet, and will not be available as a drive over USB. If you want to see that drive over USB, then remove the tick and reboot the N810, and then reconnect to USB

Obviously if you do not have a card in the external slot, then you will not see that. If you have then the best thing may be to re-format it, after backing up any data which you want to keep. Again this can be done from the "memory" dialog in Control Panel.

I actually find that the best and quickest way to transfer a fair bit of data to the removable (external) card is to remove it from the N810, and plug it into an SD card reader, and pop it back in once the data is transfered.