cdr reading - multisession strangeness

Whenever I use a cdr in my ss20 that's has several sessions written, 
(usually by importing sessions on a pc running nero-burning, or toast on a
mac)
I intially get a 'bad superblock or unknown filetype' message.
Strangely, if I then try another cdr, also with several sessions, it reads
fine, and subsequently
I'm also able to mount the origional cdr sucessfully.
I used to think it was just the old internal drive in the sparc that was
failing, but it happens
with external drives too...
I know I'm being vague about the different burning software I'm using -
should I make up
a sort of truth table of occurence with differently burnt cdrs and repost?
or is this a well known problem?
thanks - Chris db