My understanding is that k3b/Brasero are front ends to cdrtools. My system will not utilize
sysutils/dvd+rw-tools to back up files in one step. What works well for me is to generate an *.iso using mkisofs and then use the following from
growisofs(1)
Code:
growisofs -dvd-compat -Z /dev/rcd0c=image.iso
You may need to adjust your character device per the same man page.
I have not used this w/ blueray but it worked with a Dual layer dvd.