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.

-Chris

A few more commands that might be useful -

[root@nova fs]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/hdc6              25G   21G  2.5G  89% /
/dev/hdc1              15M  3.6M   11M  25% /boot
/dev/hdd1              19M  3.6M   15M  19% /mnt/hddboot
/dev/hdd6              18G   17G   57M 100% /mnt/hddroot

Also, the mount command by itself will also display the existing mounted file systems...

[root@nova fs]# mount -v
/dev/hdc6 on / type ext2 (rw)
none on /proc type proc (rw)
/dev/hdc1 on /boot type ext2 (rw)
none on /dev/pts type devpts (rw,gid=5,mode=620)
/dev/hdd1 on /mnt/hddboot type ext2 (rw)
/dev/hdd6 on /mnt/hddroot type ext2 (rw)

If you take a look at the /proc/swaps file, it'll tell you what swaps you have:

[root@nova fs]# cat /proc/swaps 
Filename                        Type            Size    Used    Priority
/dev/hdc5                       partition       136512  0       1
/dev/hdd5                       partition       130500  84100   100

-Chris