It should be monted. If a DVD has a ISO image then you need to copy as ISO
right?

You are right, it should be mounted.
But, as soon as I formatted the dvd using k3b, linux is unable to detect that
dvd. So, I can not write anything to that dvd-rw now.
Any help will be highly appreciated.

Please go into synaptic and install the restricted codecs.

Following your recommendation, I have installed ubuntu restricted extras
repository. But, still I can not get ubuntu 10.04 to detect my dvd rw.

Does the DVD fail using another software like Brasero? Is the drive itself
seen by the computer BIOS? Could it be that there is a software firmware
needed by the drive itself? If you're talking about the DVD-RW media, perhaps
another brand or if you tried + media, try - media. I know one of my DVDs can
only use + or - and I wrote it on the outside of the drawer to remind me.

Does k3b require you to clean the rw before writing? If not perhaps
just burning straight away will clear any data needing clearing for
you. Also im not at home this week but another thing I would check out
is the settings to see if you have the option to pick dvdrw instead of
dvdr as that may help. Also you could google drivers for your drive
and ubuntu.