This is what one user reports:
Card Reader – In the FCPX import window I see a PRIVATE folder and a private folder. The PRIVATE folder has three files per clip – 1 is a movie icon, 2 is an audio icon, 3 is a still image icon. Importing just the movie file brings in the audio and video flawlessly. The private folder is empty, but viewing it in the Finder it is the top level directory of the card, and encloses all the AVCHD, XDROOT, etc. items.
SD slot on back of iMac – I just see the PRIVATE folder, and it only includes 2 files per clip – the movie file and the still image file – no audio file. But again, the movie file is all that is needed for import. In the Finder all I see is the PRIVATE folder which encloses the AVCHD, XDROOT, etc. – no separate private folder.
They report that BOTH work as is though.