Found the solution to my problem.

I connected my DVD/CD combo drive direct to my notebook and its being recognized. That means my drive has no problem at all.

So, the problem is that my Firewire hub is not able to supply enough power (only 5V DC input) to power the harddisk and the DVD/CD combo drive at the same time.

However, if I daisychain my DVD/CD combo drive to my harddisk it'll work because the power is also being supplied to drive the DVD/CD combo drive by my externally powered harddisk.

Phew! Now considering getting a dedicated 12V DC Firewire hub instead of my present USB/Firewire combo hub.

So much so for power hungry devices