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after a while of my headless Pi being on it usually crashes so that I then have to psychically unplug power. I have noticed that after a crash, red light is gone but the green LED ACT is blinking at a constant rate, never stopping. What does this mean?

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The big picture here is that this is a response to critical firmware or software not being found on the first partition of the SD card.

This means the card is being read, and is formatted correctly at least to the extent that the first partition has been found. If instead the formatting is completely wrong/corrupted, the response would be the same as if no card is inserted -- either the green ACT light will be off, or else it will be on solid, depending on model (and possibly updates to/revision of the onboard firmware).

Different boards also differ with regard to the pattern occurring just once, or being repeated with a short delay. Yours sounds like the latter but the relevance is the same in both cases.

The elinux.org troubleshooting page has an evolving section about this, and since the meaning of the number blinks is prone to change and the distinction between "start.elf not found" and "start.elf not launched" is probably not relevant to most people asking this question, I'll summarize the issue further:

  • The first, small (50-60 MB), vfat formatted partition contains a handful of firmware or binary executable programs required to start the system usefully. This generally includes an operating system kernel.

  • If any of these is missing, mismatched, or corrupted, the boot will fail and a pattern of blinks used as an indicator.

The easy solution would be to replace the entire contents of the first partition from a fresh image. You could do this by burning the image to another card and copying, or by mounting the image and copying out directly.

Beware that this may produce a situation where you do not have the correct kernel modules installed in the second partition, however, that problem will not prevent the boot itself (it will mean that wifi and other things may not work), and you won't get the blinking pattern. If that happens you should be able to correct the situation by updating the OS.

The cause...

May be a serious complication, because it's almost certainly some form of corruption, and that may also have affected the root filesystem, which will be difficult or impossible to fix if you do not have a backup. However, that problem will not produce a steady blinking pattern from the ACT led, so getting there is getting somewhere (although it may not matter if you have to reburn the whole card in the end anyway).

"Corruption" of this sort could occur if the card is worn out or damaged, so if you are sure you haven't been yanking the cord too much, try a new card. It could also lead to random crashes, which is what seems to have lead you into this predicament in the first place.

  • I find corruption highly unlikely considering that I have reflashed the SD card a few weeks ago and it is still happening – user4191887 Oct 11 '16 at 21:05
  • Fair enough. Maybe it is significant EM interference, the Pi's SD card reader is damaged, the SD card is defective, there's some malware at play, etc., etc., but if you want an explanation of the meaning of the blinking pattern, there it is. There are critical files on the first partition required to boot which cannot be found or used properly. Certainly the most common cause is corruption, and having to pull the cord can do it in an instant. – goldilocks Oct 11 '16 at 21:12
  • Okay, but the blinking never stops so there is no pattern – user4191887 Oct 11 '16 at 21:15
  • If it is at a constant rate it is a pattern, and that's all you really need to know; this is why I didn't bother distinguishing 4 blinks from 7 blinks, etc. The led doesn't blink at a constant rate by accident. As mentioned above, on some models (including the 2 I have), the pattern repeats endlessly with a very short delay, meaning more or less "blinks at a constant rate and never stops". – goldilocks Oct 11 '16 at 21:18
  • You mean that the green LED blinks a certain amount of times.. waits.. then repeats? I may have to take a look again when it happens another time. – user4191887 Oct 11 '16 at 21:19
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If the PWR LED is not lit there is inadequate power. Full stop.

This LED is controlled by simple hardware on the Pi2.

Even if it is lit power may be inadequate.

Until you resolve the power issue anything else is a waste of time.

PS The Pi2/3 MAY appear to run with low voltage, as the Pi itself has an on-board regulator, and runs on 3.3V & 1.8V. This should not be considered normal operation.

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