Well, I have a PCG-FRV26 and I have had issues with the motherboard before, but I have never had an HD "not found" error. In your case, replacing the original HD did no good. The PCG series ship with a Toshiba HD (40GB, I'm guessing), and mine had bad sectors on it from day 1. In your case, I don't know why a new drive would fail, unless the bios got tweaked. Check Sony eSupport for original or updated bios. There are currently three Bios updates available for your motherboard. Here's the link: http://esupport.sony.com/US/perl/swu-list.pl?mdl=PCGFX370 installing all of these in order would definitely be a worthwhile first step.
You also might try holding down the following keys while pressing the power button to turn on your computer "ctrl+alt+f2". Hold them down until the animation logo is finished. This should reset your Bios.
If none of this works, your best bet will be to get your Sony serviced by a pro (by that I mean you will need to have your motherboard replaced.)
Hope this helps!
sewellp@yahoo.com
Patrick Sewell
January 2008
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Try fixfault.com to fix notebooks/laptop pcs.