USB Electrical Feedback

For tinyG V8, the Motor Voltage (for you, 24V) Ground Lead becomes connected to tinyG ground, and thereby USB connector shield and ground wire.

Reference Schematics can be found here (Pages 1 and 2)

https://github.com/synthetos/TinyG/tree/master/hardware/v8schematics

If I had to guess, you will find the slight potential difference ("charge") exists between the Negative lead of the 24V power supply and your work environment.
Possible reasons for this are too numerous to cover here, obviously include a defective power supply but could be as subtle as asymmetry in you incoming line voltage(240V AC ). Do you have an AC mains ground connected to the Power Supply input?
You could contact the Inventables folks, ask about how the 24V output is referenced to input ground lead.

There could be an issue in the MAC end as well.

Since you are running 240V input, I'll guess you are not in North America.
Other power related issues in your area?

A suggested test: do you have a spare in-line Laptop power supply (or similar)in the 12-30 V DC range? tinyG will function over that range, but motors will probably be starved for current so don't try to run a real job. You could substitute that supply for your 24V supply. Make sure you have the polarity correct.

Without saying, be careful. If you are unsure what you are doing, perhaps a local computer repair shop could evaluate the 24V supply output for foreign voltages.

I generally run my CNC equipment in my shop (North America, 120V) connected to an in-line Ground Fault Interrupter (GFI), which will help protect yourself, but won't "fix" a defective supply. Always a good option if available to you.

As the guy who taught me TV repair back in the 1960s said, when "working on exposed circuits, when in doubt, keep both hands in your pockets - don't touch anything".

Your fan issue - Jumpers set correctly for 12 or 24V fan? see

https://github.com/synthetos/TinyG/wiki/Connecting-TinyG

Doh, sorry, you are correct on 240W becoming 220V. I read it too quickly.

Fan - the fan voltage selector is J12 (Schematic pg2) simply selecting Vmot or 12V on board regulator, so starting without jumper would not damage anything. Do you have a voltmeter to check voltage on J11?

I am assuming that you meant the fan is plugged into tinyG, by the way.

I sort of doubt you would feel a DC charge on your laptop (even 48V hardly tingles most people)), which makes me think first of some sort of AC voltage, but from where? Do you have a Multimeter available?

You may find this item helpful as a backgrounder:

http://electronics.stackexchange.com/questions/55285/how-to-safely-ground-a-switching-power-supply-with-floating-outputs

I took a quick look at a pile of in-line power bricks in my experiment stash, a collection of old laptop, router and usb peripheral power units.
Some have two wire mains cords, with non-polarized plugs, and some have 3 wire (hot-neutral-GND) cords, all output a "floating" DC voltage.

So a possible scenario is that your MAC bricks' DC- is parasitically coupled to the hot lead while the CNC supply DC- is parasitically coupled to the neutral side, OR vice-versa.
There is no good reason to assume that the parasitic coupling is the same magnitude with the two supplies or that the phase of the parasitic voltage is the same. And, in fact, the parasitic voltage could be mostly 60 cycle harmonics or could be at switching supply frequency (typically 15KHz or higher), or a combination.

I would guess that you might get the same tingle if you were to touch the DC- lead of the 24V power supply, but the resistance of a finger touching a small screw head vs a palm resting on a large aluminum plate (your laptop case) will be quite different, as would the magnitude of the tingle.