As most similar problems, it started as a failed attempt to install Bootcamp.
Seriously, the support for Bootcamp is way below the standard.

Being unsure what I did wrong, considering that I thought I had followed the bootcamp instruction manual to the letter, I was unable to install Win7 from my disk, the installation wizard first quoting some error code I couldn't identify (I saw the error code 0x80070005 in Apple's FAQ on Win7 but I'm sure it had a 3 instead of a 7), and after I tried its own 'format' function it continued citing the partition had been formatted incorrectly, ironically enough.

I've had many unsuccessful attempts to install bootcamp over the years, so that in itself wasn't much of a surprise, but then came the horror that I couldn't start the machine up anymore.

After managing to get the Win7 disk out, all I get is a "No bootable device -- insert boot disk and press any key" error. Any key combination I've tried on startup, including the PRAM reset combination, does not appear to have any effect whatsoever. I seem unable to start up from any disk other than the Win7 one, which frankly has no options on anything whatsoever apart from the formatting options which have no effect.

This old Mac Pro I have recently inherited and getting it running again has sentimental value as well as the obvious values.

I'm using a wired USB keyboard, too.

I have been at it for a while and haven't gotten close to a solution as of yet, and am coming closer and closer to despairing, and wondering whether I've somehow managed to mess up the machine for good.
So after exhausting my own ideas without result, I turn to these forums, and humbly ask for any and all the help I can get.

I hope that there's not yet reason to give up hope.


So after several more hours of googling and attempting and mucking about, I finally broke the loop.
Eventually I found this thread, and the unplugging the other harddrives worked, and with a few scary moments along the way I now have Win7 functioning... relatively normally.

The question that remains is... how should I best go about retrying to get back into Os X at will with disrupting this ostensibly delicate balance?
Any relevant reply in the least would already be appreciated, be it only to put me at ease.
This stuff is scary, yo.