I found my lacie hdd dead with the usb casing suddenly, the casing power unit to usb was defective. It housed a seagate sata drive. With repairing on of the tvs diodes the drive itself worked again, but now after another power surge when connecting and spinning up a chip cracked and burnt. Now it is really dead and not spinning up any more. Its a seagate 7200.10 320GB barracuda with firmware 3.aad made in China.

Cause its made under OEM restrictions i guess this drive is not under warranty. So i tried to find the exact drives and changing the logic boards.

I found and tested 2 same type of drives (320 gb 7200.10_ found here in the Netherlands, one made in Thailand the other one also made in China) but still problem: i cannot access data and worse BIOS does not!! recognize the drive (connected by usb). Though the drive spins up connected with both two differnt boards, one of Thailand with the same firmware version, there is no sign of recognition by BIOS or pc.

What to do now ? I was thinking of repairing the old board with the burnt chip, the same type i found only is one the Thai harddrive logic board.

Its marking starts with

4833
ssBN then a piramid
.w76c

I'm not a DR guy, so I can't advise you on the matching of the boards, but even when replacing identical boards, you will need to transplant the 8-pin serial EEPROM chip (25P05VP) from patient to donor. This IC stores unique drive specific calibration data.

As for the burnt (?) part, it is a ...

Si4833DY P-Channel 30-V (D-S) MOSFET with Schottky Diode, SO-8:
http://www.vishay.com/docs/70796/70796.pdf

I suspect that it forms part of the negative voltage supply (-4V, -5V ???) for the preamp inside the HDA. Normally when this chip fails, you would need to test the preamp connector for short circuits on the preamp supply rails.

To help you identify the components, here are my scratchpad style notes:
http://www.users.on.net/~fzabkar/HDD/HDD_ICs.txt