This isn't a problem as such, more a request for information based on ignorance of the Linux filesystem. The very short question is:

How do I find out how much free and used space there is on the volume from which Ubuntu is running?

More detail:

I'm running Ubuntu 12.04 from a 64Gb USB3 stick, created from booting up a year-old Ubuntu 12.04 DVD and running Startup Disk Creator. The reason for this is that the Master Boot Record on my hard disk, holding Windoze 7, has gone belly-up, and whilst awaiting a recovery disk I'm running Ubunto off USB or DVD as a 'trial'. (And will continue to run Ubuntu after restoring Windoze, as I've rediscovered my love of the penguin :o))

After installing Ubuntu on the stick I ran the software update app, which downloaded some 450Mb of updates and took a couple of hours to install to the stick. A couple of times I got a message saying that disk space was short. So I looked in the file manager (or whatever it's called these days) and couldn't see the stick listed, just:

• SYSTEM
• hard disk (listed as 479Gb Filesystem)
• two other partitions that had been created by Windoze
• "4.3GB Filesystem" which when I try to open gives the error "Could not find /cow", and when I try to unmount it tells me I can't because it's not mounted - D'OH!!

Edit: screenshot of file manager

Edit: screenshot of low disk space warning

What I can't see is the USB stick from which I'm running Ubuntu. Where's it gone, anybody know?

This is tangentially related to a previous question of mine about system tools, in that I'm trying to get control and knowledge of the system in the newest incarnation of Ubuntu.