I've been having some problems with my beloved Desire HD over the last few days with the phone rarely firing up after a battery swap - the phone would only boot normally after removing battery for 20 minutes or so.
When the phone would actually boot up it seemed to work fine - SD card accessible via PC, no obvious issues.
My symptoms on boot/reboot were as follows.
Normal boot - black screen (no screen activity at all), 3 buzzes then flashing amber LED. Phone unresponsive - only pulling the battery stops the flashing light.
Attempted boot to Bootloader/Recovery (on button with down volume help) - black screen, 5 buzzes and solid green LED. Phone unresponsive.
Firing up with the phone attached to PC via USB made no difference. PC (Win7) could see an undefined Qualcomm device attached when in unresponsive mode as opposed to an Android device when actually running.
I did a fairly extensive trawl of the web, but couldn't find this combination of error codes AND no obvious boot activity at all.
I started to panic slightly as I rooted my phone and installed the CM7 ROM so all warranty bets were off and I really didn't want to replace the HD a few months before the first generation of quad core phones arrive.
Tom came to the rescue with this thread on XDA - which suggests the problem is caused by the volume rocker button shorting out.
This prompted a few cogs to turn in my rusty old brain - and realised I had noticed a couple of times since the problem started that the ring volume was set to vibrate and could only increase the volume by the software settings.
I'm not a fan of the volume rocker anyway - it gets pressed by mistake a heck of a lot more often than it gets used on purpose, but I had visions of being asked why I destroyed my Â£400 phone with a pair of nail scissors and having to answer "because some guy on the internet said it would work....."
After some hesitation I removed the battery cover, peeled back the sticker and snipped the ribbon. Needless to say, the warranty was very much no longer valid at that point - if you do this, you do so at your own risk!
I put the phone back together and fired it up - the phone now boots first time every time. Big sigh of relief!
If I need to boot into Recovery/Clockwork Mod I can do this from software (feature of CM7 ROM). Devices using the stock ROM will need to download the 4ext recover app from the Market to reboot to recovery.
One thing I did notice as I was snipping the cable was that instead of cutting it I could have just gently prised the cable/contacts out of the physical switch, cleaned out the tiny grains of sand that had got in there (a rare downside of spending Christmas on a sunny beach!) and then pushed the ribbon contacts back in - the problem may have been fixable without permanent damage.
I should add a disclaimer to say that this is NOT a course of action we would recommend. You should be comfortable with losing your warranty, permanently damaging your phone and perhaps not fixing the problem. It worked for me though!
ADDENDUM - 3rd March 2012
I've noticed a fairly high number of hits recently on this blog post - I'd like to stress that in retrospect it's clear that I should have attempted to clean out the dirt/sand in the volume rocker assembly that was causing the volume switch to be locked in one position.
Cutting the ribbon cable definitely 'solved' the problem but it should be an action of last resort!
Scribbled by Ian