Create a disk image, formatted to ext3 (or whatever)
Go into your config profile
Find an empty /dev/ubd* device – probably /dev/ubdc if you’ve got a root and a swap already. Select your new drive label from the drop-down.
Save the config
Reboot your linode
Now, inside the Linode, you can mount it like so:
mount /dev/ubd[insert device letter here] /path/to/mountpoint
/path/to/mountpoint must be an existing directory.