I have a raspberry Pi 3 b+. I downloaded Ubuntu MATE 16.04.2 and used the unarchiver on MacBook to extract the .img. I then put the image file on a USB and plugged it into a Windows 10 box and used win32diskimager to write the image to a microSD card (SanDisk Extreme PRO microSDXC Memory Card Plus SD Adapter up to 100 MB/s, Class 10, U3, V30, A1-64 GB). When i put the microSD card and power on the Raspberry Pi, the red light blinks 4 times slowly and then 4 times quickly.
If I remove the card and view it in my MacBook, I see the files there (it should boot). I get a brightly light screen on the monitors I have tried and a yellow lightning bolt shows up on the far right top of the screen.
Any suggestions? Thanks