Basically, you need an NFS server or a samba server running in your N800.
Of course, you need to make sure that your external drive is detected by the device when connected with a special usb adapter, the USB host app from extra is also needed
And probabaly you need some luck too, some drives may fail to run if they dont use an external power source.

I dont have any experience, but I'm sure you can find an answer if you search correctly the forum. (just step by step)

Good luck