At the top "file,edit,veiw etc " click on options. a drop menu will come down. In there click "settings" then click on the system tab. On that page you will find a section "temporary folders" Temp Folder and secondary. Click on the button with the dots on it next to the first Temp folder to find and choose your external hard drive. ( Just choose the drive you want as it will create a temp folder for you automatically) You can also allocate as much memory you want to the primary Temp folder on your hard drive in that same section where it says "reserve" although it will most probably do this automatically as well. After doing this you should be able to see how much free space there is displayed just above the time in your system tray.
options-> settings-> system "tab"
Hope this helps.