Thanks.
I've since fired up MediaMonkey (MM) with the Rockboxed Sansa with the 8gb card in it connected in MSC mode. The native drive and the card appear as separate drives in MM (and in Explorer) which ask for a separate sync command, I tried selecting both at once (using ctrl, alt and shift in various combinations) but that didn't seem to be possible. So it looks to me like the drives will only sync independently and so auto spanning is unlikely to work. I'll try it though - and perhaps MTP mode too.
If neither works I have another idea based on Rockbox (RB) which I think can create Playlists itself based on tags. If that's the case then I can drag and drop my stuff to the two drives and then let RB build its database (I am presuming that process does span both drives - does it?). Finally I recreate my MM playlists in RB based on the tagging done in MM. Sounds like a pfaff but it might work.
Actually dragging and dropping will be a significant pfaff (mild UK slang meaning bother, irritation, long way around to do something) because syncing direct from MM I can get 64bit conversion on the fly and auto-levelling. If I take the drag and drop route I will have to first export everything to a temp folder, level adjusted and converted to 64 bit, and then drag and drop that to the player.
Any advice is gratefully received.
Thanks.