How to make firmware of 20G HDD from 40G HDD?
First make backup of all the firmware modules and ROM. Then depop a head, remaining #0. At the same time, manually modify the model and LBA to be the one of 20GB HDD. After that, you should perform HDD Rebuild to take the above operations into effect. After that, reset the power and backup all the modules when the HDD reports on readiness. Attention: if the HDD clicks after head depop, you can first set HDD in PCBA mode (safe mode), recover the head and upload the original backup modules. Then the HDD should be OK.