Is there some risk of damaging something (especially MOBO) if I attach my graphics card outside the case?

[acaciosc]

I'm wondering if the MOBO's PCB won't have damage because of the weight caused by my graphics card (R9 280x). It's because I'm using it to mine cryptocurrencies, so outside the case I will have some extra cooling.

 

[hang-the-9]

Using the card for mining will not cause any more stress on the motherboard than if you used the card for anything else. The card does not get heavier if you use it a lot. Now if you just have the card sitting on the motherboard without a case backing to support it, then yes, that could be bad for the motherboard since the card can go side to side without any support. If you want to have it open, just keep the case open.