Most likely it is because the HDD cable is loose, as Hadakajime said. I had this annoying message for at least a month, until I decided to check it out by my own. Yes, it'll void your warranty (unless you do the opening unnoticed, there is no seal after all), but I'd do it rather than sending it anywhere for another month.
When your Archos is trying to boot, does the red HDD led go on at all, before giving the message? If it doesn't, the problem is most certainly with the communication between Archos and the hard drive.
If you decide to open your 604 WiFi, check out some instructions here
. Do the first 3 steps and you get to the hard drive. Don't worry if it comes straight out, because that might actually be the reason why your Archos isn't working.
Then just try to insert the hard drive's ribbon in it's place. Try to put it as deep as it can go, but still be very careful. When you've done that, carefully put the cover back and screw all the screws. If you're successful, the Archos will boot up and all your files are there!
So that's how I did it yesterday, it's a bit risky, yet so pleasing when you've done it right.
If you don't want to risk it at all (and you don't really have any experience in messing up with electric things), just send it to where you've bought your Archos from, and they'll do the job for you.