I just came across this posting because I too have the same issue with now power being supplied to the unit, taking the hard drive out is quite easy if you still have it. Just take out the two screws on the back (one covered with a warranty seal) and then over a mattress, pillow or thick rug gently tap the end downwards so that the inards start to slide out. Once out, you'll probably see a burnt component on the circuit board next to the on/off switch but you hard drive would be removable and readable provided that it hasn't in itself failed.
Your Dell 8400 has a SATA drive but you could find a way to adapt it to make it readable even just by replacing the Maxtor housing with a third party 3.5" IDE enclosure.
A few little pieces of advice though, in future, don't leave the any external USB device running, 24 hours, use a high quality surge/power protector (3700 joules or more (APC surgearrest)), and remove USB devices gently by either using the preferred "stop this device" utility or similar method.
I hope this is still useful after all this time, if not for you then maybe somebody else.
Best of luck
Kevin Bates