My Archos 504 will not boot up... it freezes on the Archos logo screen and never fully boots to the OS. I have tried the reset button but that does not seem to do anything. I can hear the hard drive spinning and ever once in a while I see the Archos logo flickering. The computer will also no longer recognize the unit when I plug it in via USB... Do I have a bad hard drive or am I missing something?

try to set it back to factory.
press and hold the "Volume Up", "Home", and "Power" buttons.
Release the buttons when you see the recovery screen and the Samsung logo.
Use the volume buttons to navigate the menu and select "wipe data / factory reset". ...
On the next screen, press "Volume Up" to continue.

1.Try a Safe Boot
Mac OS X 10.2 and later Safe Boot includes disk checking and repair. First start your Mac in Safe Mode. To do this, shut down your Mac. Now Turn on your computer by pressing the power key while holding down the Shift key. When you see the Apple logo, release the Shift key. After your Mac fully starts, restart your computer normally without holding any keys/buttons during startup. Note that Safe Boot is slower to boot.
2.Run Disk Utility
Turn your Mac off by holding down the power button for a few seconds. Turn on your Mac and hold down the Command and R keys until you see the Apple logo. Then you should see a "Mac OS X Utilities" window. Select Disk Utility, click your OS X hard drive and select verify/repair disk.
3.Reset PRAM
This may fix your boot failure. PRAM ("parameter random-access memory") stores some of your settings. You may reset PRAM. To do that, turn off your Mac. Now turn on your Mac. Then, immediately, hold down the Option-Command-P-R keys until your Mac restarts. When your Mac is starting up, release the keys.
4.Reinstall Mac OS X
If nothing above works, you may try this. Your Mac needs to be connected to the Internet. See number#2 above. Follow the same steps. Restart your Mac and hold down the Command key and the R key until the Apple icon appears; when you see the Mac OS X utilities window, select the Reinstall option.

After removing the battery, I discovered a small brass reset button, about 3 mm in diameter, near the battery connection terminal. This was gently pressed with a pen for 15 seconds. After refitting the battery, the unit was fully functional again. This reset results in needing to set the clock again. No other settings are affected.

hold the tv button slightly earlier than holding the power button, first tv then power (while still pressing the tv button). it should first boot up to a normal green ARCHOS screen, then it will enter the recovery mode.

I had the same problem. I droped mine and it seems to have messed up my hard drive. i replaced it today and my comp. can now reconize it. i have been trying to contact tech. support to see if it is possible to get the OS back on the PMP. if the Tech support ever picks up my call i will message back with the fix.

Please open Battery cover and PRess Small Golden button just near the HDD , it will restart the Devie

solution found!!!!!!!!!!!!!!!!!!!!!!!!!!! thank you so much for the support of the Archos team. here is what i did to repair my 504
if your "reset" button doesn't help at all, this is what you'll do.

If you power on the unit by pressing and holding the TV/LCD button and power
button simultaneously the unit will boot into a recovery screen. Once in this
screen choose "Repair Disk" and follow the on screen instructions.

When prompted, please place the following file into the 'root directory' of
the unit:

http://update.archos.com/4/a504/a504_r1.6.53/a504.aos

before i found this solution, i've already given up on my 504, but now it's back!!!! REBIRTH!!!
thanks once again to Tony H from Archos Technical Support

Kept trying the reset and just before I put a hammer through the hard drive it started again. Yippee!

try resetting it via the button under the battery cover next to the hard drive, and if not, connect it to your pc, and format it as quick as poss.

I had a similar problem remove the battery and push the gold coloured reset button. It worked on mine, You don't lose any info doing this. I got the advice from the Archos support phone number CC