I am trying to boot Arch Linux image. I've done everything according to Reinstall Manual and when I power up with created os on microSD card, the red and green LEDs are turned on, HDMI is dead, ethernet LEDS are dead. I am powering rPi from original Samsung power adapter (for cellphone), I've tested LAN cable, I've tested HDMI cable, still no activity. What am I missing? I have another rPi which is working just fine. If I put new microSD card into working rPi, I get same result as described. If I put the microSD from working rPi into testing one, it works like charm, so there are no hardware failures and neither power problems, because I tried with same Samsung adapter. What am I missing?
If the green light comes on and stays on steady, but if it only does this with one particular card and not another, there's perhaps something wrong with the card, such that there's no working MBR, or the card is in completely the wrong format.
It's not simply some corruption (unless it's in the first 1/2 kB), since in that case there would be some indication; the green light will flash in specific patterns when there is a bootloader/firmware/kernel problem.
The Arch page you linked seems to be for a generic ARM installation, which would probably result in "completely the wrong format". The pi has specific requirements, including a DOS FAT style first partition. Try following these instructions which is for prepping an SD card for the pi with Arch.
To check the MBR on linux, if the card is
/dev/sdb (note, no partition number):
dd if=/dev/sdb of=mbr.test bs=512 count=1
You should get:
testmbr: DOS/MBR boot sector; partition 1 ...etc.