I think you should first open a terminal and run mount
You should from there be able to work out if your hard drive('s partition) is
/dev/sdb1 or something like that, and the flash drive partition something
like /dev/sdc1.
Next, add the line as said by tomlukeywood, with the "/dev/sda" part
corresponding to the output of the mount command. Note that the mount points
(the folder to which the partition is mounted) has to already exist.
If you want to mount it the way you do on the desktop, which means the mount
point gets added as soon as you mount and removed as soon as you unmount, you
should create /etc/xdg/autostart/(what-ever-name-you-want).desktop, which if
I'm not mistaken should contain:
[Desktop Entry]
Name=Anything you like
Exec="udisks --mount /dev/sdb1 & udisks --mount /dev/sdc1"
Terminal=false
Type=Application
With the "/dev/sd**" part of the "Exec" entry corresponding to the output you
got from the mount command.
