Thanks but in fact both partitions do appear on the desk top and clicking on the second partition merely shows the files on that partition. Have now solved the problem by restarting the computer and clicking on the option key when the computer reopens. Then there is a choice of which partition to open.
Ah! Proverbial miscommunication.
Open a volume (partition) means to show its contents.
Boot from a volume means select it to be the Startup Disk and restart. That's done the hard way, which you described, or the easy way:
System Preferences->Startup Disk->select the one you wish to boot with and hit the Restart button.