The N810 has a built-in, non-replaceable, 2GB flash memory card. That is likely what you are seeing.

The internal memory in your N810, by default containing folders such as "Audio clips", "Documents", "Games", "Images", and "Video clips", is not mountable on your PC by simply plugging in the USB cable. Those folders are part of the tablet's own file system, and as it keeps running when connected via USB, it can't make those available to your PC as it does the memory cards.

You can use the memory cards as temporary storage to shuffle files between your PC and the internal memory of the tablet. You could also use Bluetooth to exchange files.

Alternatively, if your tablet could talk through WiFi to your PC, you could use OpenSSH server on the tablet and a SFTP client on the PC to exchange files. You may even be able to find a SFTP option for your PC that will allow you to access the remote share as if it were a network drive.

While your tablet is connected to your PC via USB, this is normal. However, while not connected, you should be able to see and access the drives via the file manager. If you are unable to, there may be a problem with the file systems on the flash cards.