MacOS High Sierra: An error occurred while verifying firmware
Apple has introduced a new file system APFS in High Sierra. The easy fix is to verify your backup and then erase the volume and run the installer against a blank / erased main storage drive.


https://support.apple.com/en-us/HT204904


Because the APFS requires support from a new version firmware, your mac's firmware will have to be updated to the latest version during the installation of High Sierra.

After the first restart, the installer prepares the latest version firmware which suitable for your mac model onto EFI system partition of your main disk with bless command. Then restart automatically and trigger the firmware upgrade process. (marked as point A)

After the upgrade process (regardless success or fail), the installer continues. It double checks the version of firmware to make sure it was updated successfully in previous process (namely point A). As a result, the installer find out that your firmware is NOT a latest version still. Hence, it reports "An error occurred while verifying firmware". Yes, it is what you saw on the screen.

The root cause of the failure at point A, in your case, is no internal drive in main bay. The firmware upgrade process only recognize the ESP (EFI System Partition) of primary internal drive. That means the upgrade process was interrupted.

So, you might have to get the primary SATA port work and connect your disk to it. It's said that, most of time, it is the hard drive cable which is out of order, instead of the chipset. If that is the case, it probably  can be fixed within $20 to get a replacement. And that was my case. :)