harddisk not found on reboot

Sounds like a thermal problem - go to Applications>Utilities>Console and in the little window on the top right of the Console window, enter "maximum temperature exceeded".

Se what that throws up.

>So I installed, and indeed it work pretty well as long as the machine does not turn into sleep mode. In case it does, it most cases it freezes.

See what comes up with Miriam's suggestion to check the logs for a thermal shutdown or sleep. But I would think that in sleep the machine would run cooler and would be less likely to freeze. So I'm not sure why the freezes are occurring then.

That you're getting the flashing folder question mark makes me wonder about the drive. I would check the drive out using the free demo of SMART Utility.

http://www.volitans-software.com/smart_utility.php

Also, get free Temperature Monitor and post back with the temps, preferably in Celsius or Celsius/Fahrenheit. Use the camera icon to post a screenshot.

http://www.bresink.com/osx/TemperatureMonitor.html

The 24's are somewhat known for drive problems due to overheating.

The drive is failing. It may have suffered from long term heat damage. The very first thing to do, since the drive may die completely at any moment, is make sure you have everything backed up, as soon as possible. You will need to replace the drive or, if you have a bootable clone on an external drive, you can continue running from there.

If the internal drive holds out long enough and you have an external, you can use (free) Carbon Copy Cloner, or SuperDuper, to make a bootable clone.

http://www.bombich.com/

I use and recommend smcFan, but don't overdo it. You want to make five settings above the defaults, each in higher increments of about 15 or 20%, up to around 2K. You want to run the fans at the most conservative setting to get the drive temp down to the range you mentined. 60-62 for the drive is far too hot. I find that it also helps to run a fan at the back of the computer. This will help dissipate the case heat. To avoid burning up the motors, I generally don't run the fans much higher than three or four hundred rpms above the defaults. And, I like to run them at lower speeds, if I can.

An external run on Firewire 800 will run pretty fast. May just take a bit longer to boot up. I know someone else who was in this predicament with a 24, but he had AppleCare and that took care of replacing the drive and the power supply, which was also failing. In your case, I don't know. In general, replacing a drive, is far less expensive than a new machine. I'd get it fixed, but that's for you to decide.

I think this may have been a particular issue with these 24s. I haven't heard of any issues with heat like this with the newer iMacs. But, I've always felt that the fans aren't ramped up quickly enough to deal with heat in any of the iMacs. Apple appears to prefer selling computers with quiet fans to making the hardware last. That said, I don't hear the fans even when ramped up three or four hundred rpms. If your fans were letting the drive into the sixties, then I think that's a likely cause of the failure.

If you run a fan at the back, use smcFan and keep the air intake vents clean by periodically vacuuming, that should help keep the heat under control.

When you bring it in, they should run the full suite of diagnostics and be able to see if the power supply is involved.

My default/idle fan rpms are different from yours, but this will give you an idea of my settings. During the winter, I run at the default, or close. Recently, we had some very hot weather here. The room ambient was around 36 and with running the fan at the back, I wasn't going higher than the "hot" setting and the drive wasn't climbing above 50. But in that kind of hot and humid weather, I avoid running the computer for long periods, allowing it to sleep and cool off more often.When it's like that, no amount of blowing hot air around is going to help that much. I try to run the fans as conservatively as possible.

The fan is a small clip-on fan mounted to the wall with a bracket to eliminate vibration on the computer desk. It is aimed upwards at the hottest part of the back, which is usually the upper left as you are facing the screen. It is quite close, about 6" (15.5 cm) from the back. If you run smcFan and a fan at the back, there will be more dust accumulation at the air intake grilles (along the bottom where the speakers are located and there may be one behind the stand, as well) since more air is being sucked in. So periodic vacuuming is really a must. Keep the area around the computer as free of dust as possible.

Did you backup your girlfriends computer before you did this?

I don't like the power supply at 72. And 75 for the drive is deadly.

Here are my fan defaults, not included above.

And, some "normal" temps in hot, but not extremely hot, weather. The computer was only being used for browsing, nothing graphics intensive. Temps will, naturally, climb up with anything CPU or graphics intensive This is from Hardware Monitor, the more complete, but paid, version of Temperature Monitor.

I entered

maximum temperature exceeded

in the Condole and nothing showed up. Hopefully, this means mine has not exceeded the max temp.

I use the smcFAN at the default setting until things get warm then bump it up for while.

The hot spot in my iMac is the airport card