from http://bugs.musicbrainz.org/ticket/4893 (has quite a few comments)
Would be nice if Picard would figure out by himself which device is the CD device
to which I said:
libdiscid can already find the correct device by itself in OSX. The problem is mostly just that Picard disables the CD lookup button if there's no device set.
It could be a problem if you have multiple drives with CDs in each one, but as I said:
I think people can live with ejecting one of the CDs in the rare cases that they have both and have CDs in both. Far more usable than having to figure out which device to use.