I suppose W800c is equivalent to D750i/K750i/W800i ? And your phone is totally dead?

If both assumptions are true I'm pretty sure you can repair your phone with DVT12.7 (works with 'standard' DCU-60 cable) or DVT13.8 (works with Cruiser 4in1, which I suppose you don't have right now). Try to write K750_EROM.DVT with DVT12.7 maybe it does the trick. You can download that file from DVT server. After flashing try again your phone. If it still doesn't work try the "Recover Phone" function in DVT13.8 (but you need a cruiser cable for that afaik).

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!!! csloader refused to start fs services,errcode: 1  !!!

I have the same error last night! i just flash with DVT client 13.8 as main file K750_EROM.DVT and then finalize with sonics and it works for me! 

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!!! writing full signature failed, errcode: 3  !!!

 i wride GDFS back up in sonicks with function write GDFS and everything is ok:)

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