Won't boot without HDMI connected (solved)

[jonrogersuk]

I've been using my RaspberryPi headless, connected to a USB hard disk and the network from the cupboard where my router lives. It was running for 21 days like that, no problems, then I re-started it. Now, bizarrely, it will only boot when I take it into the living room and connect it up to the TV with HDMI. I have tried all the available power supplies, so I don't think it's that, and simply un-plugging the HDMI cable is enough to stop the RPi from booting.
When I plug in power, the red light comes on, then the green OK light flickers for a couple of seconds. If the HDMI is not connected, that's all it will do. Network lights don't come on, I can't get anything to tell me what's happened. With HDMI, the network lights come on after a tiny pause and the green OK light keeps doing its thing.
I have tried different SD images, no good. I have tried booting with nothing connected, no good.
What can I do? It's not practical to keep the RPi where it is currently working - any suggestions? :

[dom]

Have you tried rpi-update. There has been a fix for a problem just like this.

[jonrogersuk]

Amazing! Thanks, that was exactly what I needed - you deserve a medal! And if that was post number 1337 for you, even better!

[Mark]

I am having the exact same problem on a Pi I just received. I've run rpi-update but the problem persists.

As with the OP, everything works fine if I boot with HDMI connected. I'm using the latest Raspbian, updated, if that matters.

[dom]

@Mark
You could try:
boot_delay=1
or
hdmi_force_hotplug=1
in config.txt.

[Mark]

hdmi_force_hotplug=1 seems to resolve it. Thanks!

[KenT]

There are a number of people with this problem. Is there any downside to configuring the software release to hdmi_safe being on, or at least hdmi_forcehotplug?

Once people have got their system going they can always change to unsafer options.

[dom]

There are a number of people with this problem. Is there any downside to configuring the software release to hdmi_safe being on, or at least hdmi_forcehotplug?

Once people have got their system going they can always change to unsafer options.

hdmi_force_hotplug forces hdmi output. Someone with a composite TV will get nothing.

[wymand]

I also had the no boot without HDMI problem and tried all the fixes listed to no avail. Then I noticed in rasp-config advance options I had enabled the GL driver. Once the GL driver was disabled, the pi boots fine without HDMI