However if you are using a mains powered workstation then the casing of that machine and possibly the USB shield will be connected to "true" electrical earth.
That is only true for some classes of equipment:-
In general, there are a two classifications of portable appliance used on the normal mains supply, Class I and Class II. Class I appliances usually have metal casing and must be connected to an earthed supply. Class II (also known as double insulated) have two layers of insulation to prevent a fault causing an electric shock. Class II appliances are marked with the "Double Square" symbol and do not need earthing.
Does this sort of testing only apply to something with a power supply
Yes it is called PAT testing.http://www.pat-testing.info/
I tried soldering onto the Al,
It can be done but you need special solder, it is very expensive at about £35 per meter.