The firmware revision for ND-6750A is 2.P1.
There are two parts for this firmware flash. You need to flash "Boot" part first, then flash "Main" part of the firmware.

[Boot flash]
-Unzip "6750BTR.ZIP" and copy extracted files to a DOS system.
-Plug the drive to the system. Boot the system and execute "6750BTR.BAT".
-When the firmware flash is completed, switch off the system.

[Main flash]
-Unzip "NECND6750_v2P1.ZIP" and copy extracted files to Windows system.
-Boot the Windows system and execute "NECND6750_v2P1.EXE".
-When the firmware flash is completed, switch off the system.

This is the end of firmware flash process.