As for the backward compatibility, SATA II supports SATA I interface, but there's a
limitation on the capacity of the SATA II hard drive you're only allowed to use.
Some Laptop brands with SATA I interfaces can support up to 500GB and when
you installed a SATA II with the same capacity it will just function like a SATA I
(as far as speed, write, cache times is concerned). BUT, lets say you want to
upgrade your SATA 1 500GB hard drive to 1 1TB SATA II interface?.
Well, before doing this, make sure to read your laptops specifications whether its
motherboard can support or can detect large capacity hard drives (1TB or higher)
on a SATA II interface, becaue if your laptop's motherboard cannot support very
large capacity hard drives like those i mentioned and you already bought a 1Tb
hard drive and installed it in your laptop for an upgrade, chances are, no matter
how you tweak, configure your laptop's BIOS it can never detect or format the hard
drive you just installed because it cant support it.
The best thing to do is read your laptop's manual or go to the manufacturer's
website and look for the model of laptop you're using and read its specs, if you find
there's a limit on the capacity of SATA II hard drive you can upgrade or it can
support, then you just have to follow the limitations. If its 1TB only on SATA II,
then 1TB Sata II it is.