Imac 21.5 white screen on boot up

I have the A1311 2009 21.5" iMac. The computer was running fine. I changed startup disks to boot off of a friend's external USB backup to get some files off for them (I needed to export their address book and get other various files)
When I finished getting what I needed under system preferences I changed the boot up drive back to my imac's internal drive and clicked restart.
The computer restarted gave the startup chime and stayed at a blank white screen. No spinning sprocket.
I am using a usb keyboard and usb mouse.
The only boot keyboard command that works is command+option+p+r and when I do that I get the chime and a reboot.
I've tried resetting the SMC by unpluggin it.
I've tried inserting a disk warrior cd to boot off of it, but it doesn't boot. And holding the mouse button on startup does not eject the CD.
I've tried holding c when booting to boot of the CD drive.
I've tried holding option when starting. I've connected my own bootable USB backup and held "option" when starting.
I've tried holding shift when starting. I've tried command+r when starting.
Nothing happens. I always get the chime and then a white screen.
Looking online, it looked like several macbook air/pro owners with a similiar problem disconnected their HDD and then were able to boot off an external source. So I took off the glass and LCD and disconnected the 3 hdd connectors, put the screen on and tried booting again using all the steps above and every time always the same, a chime and then white screen.
I took all the ram out and when it was turned on got a low intermitent beep and black screen. Put the ram back in and get the startup chime and white screen.
I'm at a loss as to what else to try. I can't see how booting off my friend's external usb hdd would lead to this.
Any suggestions.

I reopened the imac, removed the optical drive (hdd was already out). Restarted with my USB external HDD connected, nothing happened, white screen still. I then reconnected the friend's USB HDD also and restarted and got the screen to choose a startup disk.
Computer then boots fine from my external hdd. Turn it off, reinstall my HDD and optical drive.
Turn it on again and am greeting with the same blank white screen again.
I then remove the optical drive again and am able to boot from the internal drive. BUT when I go to system preferences to change startup disk, if I check the internal drive or the external USB drive and click reset I got the error "building boot caches on boot helper partition failed". I have no idea what is going on. I repair permissions several times.
I then restart the computer several times and it seems to restart consistently from the internal drive. I then put the optical drive back in and it now appears to be working.
The original external drive I booted from was a 10.5 macbook pro cloned drive. So I don't know if that somehow corrupted my 10.8 imac drive or some firmware that made it such that it wouldn't boot.
What a hassle.
And for anyone who cares and has read this far, I am living in a remote town in Mongolia at the time and didn't have any suction cups to remove the front glass from the imac. I do have an iSesamo tool to open up ipod touches and such, and was able to use that to get the front glass off.
If anyone has an idea of what happened I'd be interested in hearing.

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I reopened the imac, removed the optical drive (hdd was already out). Restarted with my USB external HDD connected, nothing happened, white screen still. I then reconnected the friend's USB HDD also and restarted and got the screen to choose a startup disk.
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I then remove the optical drive again and am able to boot from the internal drive. BUT when I go to system preferences to change startup disk, if I check the internal drive or the external USB drive and click reset I got the error "building boot caches on boot helper partition failed". I have no idea what is going on. I repair permissions several times.
I then restart the computer several times and it seems to restart consistently from the internal drive. I then put the optical drive back in and it now appears to be working.
The original external drive I booted from was a 10.5 macbook pro cloned drive. So I don't know if that somehow corrupted my 10.8 imac drive or some firmware that made it such that it wouldn't boot.
What a hassle.
And for anyone who cares and has read this far, I am living in a remote town in Mongolia at the time and didn't have any suction cups to remove the front glass from the imac. I do have an iSesamo tool to open up ipod touches and such, and was able to use that to get the front glass off.
If anyone has an idea of what happened I'd be interested in hearing.

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Imac 21.5 white screen on boot up

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