A Axis showed up out of no where

when control asks the Blackbox who it is, the BB is telling it that it has an A axis.
so, either
it is lying, ie a USB communication error, turn everything off, unplug/replug all the cables, move power cables away from the USB cable, turn everything back on and see what happens
or, the XYZA firmware got flashed to the blackbox, and it really thinks it has an A axis.

 

you need a Blackbox X32
in CONTROL you use the wizard to flash the XYZA firmware.
exit CONTROL
reset the BB
start CONTROL and it will show the A axis.

Note that now the Y output on the BB is Y and the Y2 is now the A axis.
you must wire the 2 Y motors in parallel to the Y output, and only the A axis to the Y2 output.

 

Not surprising, clean start, checks with controller how many axes it has

EMI: See

EMI causes serial corruption, messes with the parser, - the data travelling between BlackBox and the PC updates every 100ms or so, and the EMI messed with the reliability, flipped some bits - it sees an A instead of X,Y,Z due to corruption of the data stream, and (as it should) displays the A-Axis stuff because the position report said you had an A (even though you don't - but EMI messed with the text): docs:blackbox-x32:faq-emi [OpenBuilds Documentation]

When its an actual 4-axes machine (or 3 axes lathe or other config needing an A-Axis)
1) Have a BlacBox X32
2) Wire the A-Axis to Y2
3) CONTROL > Wizards and Tools > Firmware Flashing tool > BlackBox X32 Tab > Machine Style > 4-Axes > Flash
Now the position feedback stream contains A-positions so the frontend will show the A-axes DRO and Buttons and the firmware accepts A-commands to move the motor attached to Y2 (Y2 is the reconfigurable spare axis, defaulting as a ganged-Y, but can be set to anything)

 

Exactly. The same EMI that corrupted your serial stream to the point of showing non existant axes, is also corrupting the stream causing stoppages and random moves and all kinds of other problems. Fix it.

I

 

replace your USB cable with a better quality, shorter one.
follow ALL of the EMI recommendations in the docs, not just some of them.

if you are running from a laptop, unplug the laptop power and do a test run. just make sure the laptop does not powersave during the run by moving the mouse every 30 seconds or so.
if you get a successful run it points to the laptop PSU as the problem. try putting the 2 pin plug in the other way round. (2 pin plugs are evil, I replace all mine with 3 pin)

if you have a VFD running your spindle, then that is the problem and you will have to do all the EMI preventions things, they are electrically noisy (even good ones).

Even LED lights can cause problems, back when we had free to air TV our LED kitchen lights would put one TV channel into snowy interference, turn the lights off and the picture was sharp.