Frodeaux has posted a guide here:
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Disregard any mention of "Original Spoofed Hitachi Firmware" as your's isn't spoofed so just follow the instructions and use your original hitachi firmware.
Keep in mind that the Hitachi's MUST be restored first though before flashing.
I've used Frodeaux's guide and altered it for the steps you would take.
DIRECTIONS:
A: Restore Original Hitachi Spoofed Firmware
1. Launch Firmware toolbox version 4.8
2. Load Original Hitachi Firmware
3. Click tools->Direct Drive Flash->Differential Flash (RESTORE),now click read and detect differences. At this point, it will ask you to insert a DVD movie or an original 360 game. Once you do(give it a few sec to spin up), click ok. Choose your drive in the top drop down menu if it isn't already.
4. Click Start Flash
5. At this point the drive should have original firmware in place, TEST the drive ( should only read real disks and not backed up disks )
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B: Make new iXtreme 1.51 Firmware
1. Launch Jungle Flasher 1.51 Beta
2. On the " FirmwareTool 32 Menu " Select " Load Source Firmware " and select the original Hitachi firmware.
3. Next select " Open Target Firmware " and select the new iXtreme 1.51 Hitachi firmware file. (correct version for your drive)
4. Select " Spoof Source to Target "
5. Select " Save to File " to save your new iXtreme 1.51 firmware file
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C: Flashing ixtreme onto the Hitachi
1. Launch Firmware toolbox version 4.8
2. Load the new firmware file you just created in Jungle Flasher
3. Click tools->Direct Drive Flash->Differential Flash (PATCH),now click read and detect differences. At this point, it will ask you to insert a DVD movie or an original 360 game. Once you do(give it a few sec to spin up), click ok. Choose your drive in the top drop down menu if it isn't already.
4. Click Start Flash
Courtesy of Frodeaux.