Well here is my not very scientific feedback on my own findings.

Copied ebay discs are a lottery in the MKIV unit.

I suspect that the main two reasons are:

1) Quality of the physical media = poor/cheap
2) You have no control over how it was burned or what was burned (source data). = Poop in = Poop out.

Firmware:

The MKIV was not designed to run the latest v32 "hacked" firmware with Speed Canera Alerts, 7 digit Post Code - Day & Night Mode etc:

1) It simply overloads the system as it continually reads from the disc in an effort to keep the latest image on the screen.
2) This results in the head unit freezing with a variety of errors. The most common being "unable to read the disc" That is not a lie, It is actually unable to read the disc.

TMC:

This places a massive load on the system as it is constantly being updated via the radio then converted into map data, then displayed. As you enter built up areas, this update process swamps the nav system that is also attempting to deal with all the other "hacked" updates. = The system re-boots and keeps re-booting as it cannot keep pace with all the data being thrown at it.

Solution (this worked for me)

1) Use only OEM discs (firmware and Map) no hacks.
2) Turn off TMC (it is a great feature but is a killer when it gets busy)
3) Give up and use a TomTom or Garmin - Better maps and more reliable + touch screen and speed camera's etc are all in there.

I want the OEM screen and do use it for radio, music etc. But the Map system is antiquated and prone to hangs (just when you need it most) and that is with OEM discs and Firmware.