Hi! I've bought this screen http://www.robotshop.com/en/28-touch-lcd-display-shield-arduino.html (waveshare SPI), which has an micro-SD slot, and connected it to an Arduino Mega. When inserting a micro-SD white and blue dots appear on the screen. The screen is well functioning otherwise. Anyone who has an idea of what might be the problem?
Why not post your sketch? Or attach it?
Just post links to non-standard libraries. Then people might try to reproduce your project and see any problem.
The beauty of Arduino is that anyone in any part of the world can contribute and help. Ok, not everyone has your particular TFT, but they probably have a Mega2560. And they definitely have fresh eyes.
These displays are not designed for "live" insertion/removal of an SD card.
The problem is that the screen and SD Card share the same SPI bus and when a card is inserted it briefly messes up the data that is being sent on the bus or injects rubbish on the data lines which happens to get interpreted by the display as something to plot on the screen.
Since the bus is "live and powered" when you plug in the SD card you risk corrupting the SD card and making it unreadable, so then it has to be reformatted to work again.
So the behavior you see is "normal" due to way the hardware has been designed.
The correct way to use these displays is to power them down before inserting or removing the SD card.