It's also possible to decode which part of the boot process the Raspberry Pi is stalling at. Here's a list of what the various flashing modes from the ACT/OK LED mean.
In use, we've found it easier to ignore the flashing status unless you're prepared to delve into the complexities of the boot procedure, and while this can be fun, it's also frustrating if you just want to start playing with your new device.
3 flashes: start.elf not found
4 flashes: start.elf not launched
7 flashes: kernel.img not found
8 flashes: SDRAM not recognized. You need newer bootcode.bin/start.elf firmware, or your SDRAM is damaged.
For devices made before October 2012 or old devices there have never been updated:
3 flashes: loader.bin not found
4 flashes: loader.bin not launched
5 flashes: start.elf not found
6 flashes: start.elf not launched
It is possible to troubleshoot these problems by looking for the files involved and making sure they're correct. Take a look at something called a checksum and make sure the checksum for the files on the card is the same as the checksums for the original files.
As these read errors are more likely to be an indication of either the SD card not being read correctly, or the Raspberry Pi operating system not being written correctly, we'd recommend trying to get hold of a new card first and writing the image with a different method.
Our Raspberry Pi installation guides for Windows, Linux and Mac OSX covers three different operating systems, so it might be worth trying a friend who uses something else. However, we have experienced problems with several SD card readers – especially those embedded within laptops and netbooks – and we'd suggest switching to a standalone reader first to see if that helps.
These are available cheaply, but they can often be found bundled with the SD cards themselves. If you've got your Raspberry Pi connected to a display, recent versions will also show a kaleidoscopic boot screen.
If it makes it past this and no further, then the problem is once again in the hands of your power supply.
Even when booting sometimes you won't immediately get a video signal. The Raspberry PI (and its Raspbian OS) is designed to output a HDMI signal, but if it doesn't detect a HDMI device connected it will default to generating a composite signal on the RCA port (or 4-pins 3.5mm A/V jack on model B+).
That sometimes means you have to turn on your monitor before booting. It may even mean that the monitor must be switched to HDMI input. It depends on the monitor. Booting NOOBS works a bit differently, as it will always output a HDMI signal, (even if you have nothing connected to the HDMI port) unless you press one of the numerical keys 3 (PAL) or 4 (NTSC) to switch to a composite video output mode.
Networking
If your Raspberry Pi has been able to successfully negotiate the early boot procedure, the operating system will start being read off the card in earnest. Very soon after Linux starts to boot, there will also be a flicker of the fourth LED (LNK), followed by the remainder of the networking LEDs lighting up about half a second later – these LEDs on newer models are found built into the network Ethernet port.
This happens as the networking stack is initiated and forms a link with your Ethernet network. The status of these LEDs is exactly the same as the LEDs on the rear of nearly any other Ethernet port, and are more likely to indicate problems with your network than with the configuration of the Raspberry Pi itself.
The orange LED indicates a full duplex connection. The green LNK LED is the networking equivalent to the ACT LED for SD card access, because it's this that flickers on and off as data is received and transmitted across the network.
If the LED does light up, you can assume, at least at a low level, your hardware has been able to negotiate a connection between your hardware and the switch that manages your network. But this doesn't necessarily indicate a working network connection where you can browse the web or check your email.
If you've got activity but no working connection on your Pi, you should look at your local network configuration, first by making sure your Pi is getting an IP address from your router, and then secondly from the Raspbian operating system itself.
It is able to configure itself under almost any circumstances, but there are sometimes problems if your network is a little weird – you could be using two domains, for example, or a couple of routers. The only way to solve these problems is to try to connect your Raspberry Pi to the simplest and most visible part of your network and then work your way back to the configuration you'd like to see.
The final LED is used to indicate the speed of your network. If it's on, then your Pi has negotiated a speed of 100Mbps. That's 1,000,000 bits of data per second, or 100 megabits, otherwise known as a Fast Ethernet connection.
This is a step up from the very basic 10Mbps indicated if this LED doesn't light, but it's still some way from the current fastest standard, which is 1,000Mbps, also known as a Gigabit connection.
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