This is my first post and I hope it is a good one for you! I have lurked at this site for a long time, and I have found the site very useful. Hopefully this discovery I have made will be of use to others!
I have figured out how to crossflash an HP DVD740 so that it becomes a DVD840. They are identical hardware-wise…the only thing holding the 740 back is its firmware! If you go to the HP website you can search to find the firmware for the 840 drives. When you try to upgrade the 740 to the 840 you will get a message that the drive model is not correct. It will not let you upgrade.
Here is the fun part. We all know that the HP drives are actually 4166B’s. Search the web to find the firmware upgrade for the GSA-4166B. In the RAR folder will be a firmware flashing utility. This is the part you need. Now go back to HP and download the firmware for the DVD840. Save it to your hard drive and right click the file. Unzip it with WinRAR. Go to the folder you unzipped to and find the .bin file.
Run the firmware flashing utility from the 4166B download and use the .bin file that was created when you unzipped the HP file that you downloaded. It will flash your DVD740 making it the DVD840. This works on both the internal and external versions of the HP drives! Just be sure to download the correct file from HP (internal or external).