Hi everyone, I have already successfully recovered my data without RS232 TTL or using Hyper Terminal. I did it using an identical HDD which was a ST3500320AS, SD15 Thailand.
Ok, I successfully recovered my HDD (ST3500320AS, SD15 Thailand) with the help of my friend who has a working (ST3500320AS, SD15 Thailand). I told him about Seagate having firmware issue so he was afraid and he when to update his HDD. He was using MooseDT-SD1A-2D-8-16-32MB.ISO from hxxp://seagate.custkb.com/seagate/crm/selfservice/search.jsp?DocId=207951 (Change hxxp to http).
So now his HDD is using the SD1A firmware and working fine.
Below is quick reference that I'll be using in my guide.
Y: Bad HDD (SD15), Bad PCB (SD15)
* Z: Good HDD (SD1A), Bad PCB (SD15)
* Place the Bad PCB on the updated HDD (SD1A, remember my friend's updated HDD)
Connect Z to your computer but do not let BIOS detect the HDD by either disconnect the SATA or power cable or in my case using external power cable to turn on/off the HDD. If Z is connected and powered up, your HDD light will permanently stay on and may stuck at the BIOS screen.
Make sure ONLY this HDD is connected to the computer. You won't want this firmware to screw up your other HDD.
Insert the SD1A firmware CD and make sure BIOS boot from it.
After the Firmware menu appears, reconnect Z and power on, and press 'S' to do a search for the HDD. When Z is powered up, you will hear clicking sound but its ok just let the firmware do the search.
After some time the search will fail to find any matching HDD and returns to the menu screen but the clicking sound of the Z is gone.
Do the search again by pressing 'S'. This time it will detect Z and return to the menu screen.
Now, depending on your HDD model (I press 'A' for mine), press either 'A','B', or 'C' and cross your fingers while the update takes place.
I waited about 10-15 minutes then message stating that firmware update was successful appears.
Shutdown the computer and put back the Bad PCB on the Bad HDD thus making back Y.
Boot up your OS and pray that your HDD is working again. Cheers!
You may try different approach from the steps that I took but this is what I did and it worked for me.