Strange Big Firewire Icon on Startup

Hi! Sounds like it's trying to start in FWTDM. Try going to system prefs and reselecting the correct startup disk. tom

Hi! Try resetting the pram as per THIS. Tom

Just to make sure there are no issues with the startup volume and give it a clean bill of health, do a Repair Disk, Repair Permissions and Check for Preference File Corruption.

Repair Disk: boot from the installation DVD, once past the language selection initial screen, run Disk Utility off the Utilities menu and do Repair Disk. If any errors appear, repeat until none. If unable to get no errors, you will need something stronger, like Disk Warrior.

Repair Permissions: boot normally, run Disk Utility off the startup volume, do a Repair Permissions. Beware, Leopard takes an awfully long time to finish this. You don't need to repeat.

Check Preference Files: get a third-party utility such as [Onyx (free)|http://www.titanium.free.fr/pgs/english.html] or [MacPilot (shareware)|http://www.koingosw.com/products/macpilot.php]. If using Onyx, follow the path Verify/Preferences. If using MacPilot, follow the path Tools/Disk & Files/Verify Preferences Files Integrity.

You can also try purging all caches, user and system. Also, how old is the iMac? Cause the internal backup battery may be dead or dying, causing PRAM corruption.

All those suggestions suspect the system but if you try them out and they aren't helping, I'm going to start suspecting the hardware.

For example, sometimes funny things happen and I realize that they are all normal results when the Command key is pressed as a modifier. That tells me that I might have a stuck Command key, so I bang on it a few times and blow some air under it and a crumb comes out and everything's OK.

FireWire Target Disk Mode is invoked by pressing the T key. You should investigate whether there is any debris affecting the T key and causing it to actuate at random. It is also possible that the T key is receiving a spurious software signal, but because this is happening at startup before the system loads I sort of doubt that.

If you go poking around under the T key just be careful not to break the key off.

Wow. If the broken keyboard was wireless, it was probably using up more battery than usual due to the unwanted keypresses during idle time. Good thing you fixed it.