I have tried to install the operating system 12.04,13.04) on an 8G memory stick. It installed ok except when I booted from it, it said it only had 3.5G total and was full. When I tried to reinstall it with the live disc, it said it needed at least 4G to go ahead and that box was ticked. It seems as if it doesn't want to provide any memory not being used as the operating system. How do I ensure that the full memory is available and read/write enabled
The default maximum size for your casper partition (persistence) is 4gb, and you can set this pretty easily if you use a tool like UNetBootin. Just set persistence to the maximum of 4gb. But if you want more storage space, select no persistence. Then you can use a partitioning tool to create a casper partition to use the full 8gb of your flash drive. Here is a good tutorial.
Like others have said, it may be that you just set your live usb into persistence mode when you made it. Some usb creators make it hard to see graphically whether or not persistence mode is enabled.
I prefer Linux Live USB creator as far as looks and ease of use goes, it's also really easy to make sure you are/are not in persistence mode when making your live USB. However, UNetBootin is definitely better when it comes to Linux Distro compatibility. I don't know which USB creator you're using, but if your'e making a 13.04 usb I'd recommend Linux Live USB creator.
Linux Live: http://www.linuxliveusb.com/
Be sure when you make your Live usb that you always format it, both of these creators have the option to do that during the process. That should also guarantee that you have the extra free space that you're missing.
I hope this helps, good luck!