> You are right, it should be possible.

   Which "You"?  Which "it"?  What are you talking about?

> But it doesnt work for me, i get "invalid media format". I have tried
> at least 2 CDs with no success (even when the drive is visible in the
> ouput of show dev).

   With my weak psychic powers, I have no idea what you did, or what
happened when you did it.  Among the other things about which I have no
idea are:

   How you got VMS installed on what.

   Whether your CD (-R/RW?) drive has ever worked for any purpose on the
VAX.

   How hard you want to work to avoid a couple of minutes worth of
typing to get two PAKs enabled.

> [...] I have more drives, but the others do not support 512 byte
> blocks. [...]

   I also have no idea what "the others" are, or why you believe that
they can or can not support 512-byte blocks.  Many Toshiba drives have
worked for me on VAXstation 3100 (30/38) systems over the years.  Some
had block-size jumpers, some did not.

   As usual, showing actual commands with their actual output, or actual
model numbers (or any actual information of any kind), can be more
helpful than vague descriptions or interpretations.  Asking your readers
to guess what "it doesn't work" actually means is a good way to lose (at
least one of) your readers.