i am a complete newbie at Arduino and it seems I am learning the hard way.
I bought an Arduino Mega 2560 from a UK seller which looked to all intents and purposes to be a genuine Arduino. (Correct gold coloured marking and Arduino looking on bottom). Pretty sure it was not as I only paid £8 for it.
However it was working until I connected a motor shield fed by a DC adapter. As the USB was still plugged in to computer, computer went off due to, I suspect, an overload via the motor shields DC adapter back-feeding into it.
I will have to start doing more reading up before delving.
Apparently there is something on the base I should have scraped away to stop the back-feed of supply to the Arduino. Perhaps someone could clarify exactly what needs removing and where.
The event left the Mega with the TX and RX LED's constantly lit in (YELLOW) but I do not know what this means.
Thankfully the computer re-booted without any problems.
I acquired a second Arduino (this time an UNO from the UK - different suppler) and that is functioning (hurrah).
In the meantime to replace the Mega I ordered one from China which eventually arrived but was not functioning.
Finally getting to the point, (sorry for the waffling) but I am left with one Mega with the TX & RX lit continuously (YELLOW)after the mishap and one mega that "out of the box" when plugged in has R17 LED flashing (RED) and R18 lit constantly (RED).
My Windows 10 does not recognize either Mega even after using the Arduino UNO to re-boot-load both.
The only thing to add is that after boot-loading, the UK purchased Mega (motor shield event), as well as the 2 x solid lit LED's the one to the side of them now flashes (YELLOW).
Q1) Can some kind person confirm that the non-working Mega's may as well be binned.
and more importantly -
Q2) Anyone advise me on this prevention of back-feed to the Arduino when driving shields with another power supply.
Many thanks in advance. 
There are LOTS of different motor shields and some require a correctly placed jumper or even a cut trace.
Have you tried the LOOPBACK test on any of the boards (SEE HERE)
What do you see in DEVICE MANAGER (pics would be nice)
Have you installed the CH340/341 drivers (often needed for clones but not part of the install package)
If you managed to shut down the computer with a motor shield, and now the lights aren't lighting up the way they did, place the mega in the trash, and go pick up another one. At least you only burned 8 quid (and - that's a bottom-tier clone, at that price - I wouldn't rule out questionable cost cutting measures as a contributing factor here).
There are some really dubious motor shields on the market. The story you've posted "using motor shield, things seemed okay, then something bad happened while plugged into USB, and now it's dead" is pretty common. Usually it doesn't hit the computer though - you should seriously figure out what happened before you try it again - and I'd put a powered USB hub between the arduino and your presumably expensive computer for good measure.
Thanks for the replies guys.
I really appreciate your time and knowledge.
With regards to the problem Mega's, I will try the drivers and the LOOPBACK tests, as suggested, do another bootload and report back as soon as I can.
I have now noticed a jumper on the motor shield PWR which can be removed. I assume that negates any feedback to the Arduino. Actually just established that the jumper needs to be in place to feed a DC source to the motor shield when it is not stacked. However the jumper makes no difference (on or off) when stacked and Arduino is plugged into the computer via USB. I.e. With no external supply to the shield, the USB is feeding the shield as well. So I am still rather confused as to what I should do when I need to feed a few small (say 12v) motors fitted to the shield and not have that back feed to the Arduino thus damaging it. Any suggestions please?
I can now report the following which still leaves a query.
I have successfully loaded the CH340_SER drivers (Screenshot 1) and the computer now recognizes the Mega from China.(See Screenshot 2)
I have done a LOOPBACK test on this as well and that echoed back OK. (See Screenshot 4)
I have also been able to load up a sketch (BLINK) and it has worked on said Mega.(Screenshot 3)
I have even then successfully edited the sketch (extended the delay ON and OFF) and the result is evident where I see a flashing LED and a steady one. Obviously, the flashing one is the one that changes as I alter the sketch.
However both LEDs are red. No Green or yellow LEDs light up anywhere on the board, unlike when programming the UNO. (See other 2 photos)
Can anyone explain this please.
The damaged Mega is not recognized and will be going in the bin.
Regards,
Different clones may use different LED colours is the best I have as it looks like you have a working board.
If you have a programmer the damaged one might be worth checking to see if you can program it using the ISP method before you consign it to its fate.
Hi Ballscrewbob,
I already tried to re-Boot-load it after I Boot-loaded the one that is now recognised.
It is not recognized either by a Windows 10 or a Mac whereas it was working at first, before I messed up with the stacking of the mega Motor Shield board mentioned in my O.P.
I take on board your point about the other board functioning and I should just accept it "as is" and utilise it.
Thanks for your input.
Delboy.
Sorry I may not have been clear but sometimes its possible to upload an actual sketch using another programmer on a faulty board.
The bootloader may be the issue but sometimes that can be overcome...Worth throwing a modified blink sketch at just as a final test using an external programmer to be sure.
Thanks for the insight.
Unfortunately, I do not have a programmer but will bear that in mind and perhaps just shelve the faulty Mega until I acquire one.
Regards,