max for live crashing with external

hi,
Im new here... so may be an obvious question :o)
Ive developed an external which works fine in max 6, and seems to work fine in Max for Live when I initialise it, but when I save it immediately crashed both Max and Live.
Then later I can reload it in Live, it appears to be saved ok, but if I attempt to edit it, it crashed Max.
I suspect its an issue with Max trying to create a second instance of the external , but quite hard to debug, as I get no logging etc....
any thoughts about how you deal with this etc.

... it is true that my external should be a singleton, as it accesses a USB device, so two instances will not work... but unsure how to prevent this in this Live instance.
(since in Max you know you have one instance, where in Live you appear to instantiate one in the Midi Effect, and then when you edit it, you appear to create a second... i.e. recreates the patch rather than edits the existing one)

any help appreciated, as im new to this environment... and lots to learn :o)