After hours of researching if it's possible to upgrade the MyBook World II with bigger hard drives i finally just tried it.
Around one month ago i bought a MyBook World II 2x1TB and ordered two WD 3TB Caviar Green SATA drives.
I did not have any data on the drives, so if you already have data on your drive it would be a good idea to backup them somewhere else. But no panic, if the upgrade won't work you can still use your old drives... but your data on the drives will be lost.
Drive A = Original 1TB drive in bay A
Drive B = Original 1TB drive in bay B
Drive C = First new 3TB drive
Drive D = Second new 3TB drive
Update to latest firmware
Reset to factory
Power down & Exchange drive B with drive C (first new 3TB drive)
Power on & do a Factory reset
Check if the MyBook boots and is working (reachable through the web interface etc. )
Power down & Remove Drive A & C
Put drive C into bay A
Put drive D into bay B
Power on and do a factory reset
You should have a new 2x3TB WD MyBook World II now
If this is not working for you, you can still Remove drives C & D and put back A & B.
The MyBook World should boot as normal.
Some notes for compatibility and product warranty:
In some forums and communities you'll find the answer that this procedure is not supported and it's not allowed to put larger drives into the MyBook World. My experience was that it works, anyway, the product page of MyBook World says:
"[...] Want to upgrade a drive? Simply open the case and replace the existing drive [...]"
Which includes the word "upgrade"... In my understanding this procedure shouldn't void the warranty if you're using WD CaviarGreen drives.