On the emulator, I can unmount the SD card from the Settings.
On OSX, I can then mount it using hdiutil, then unmount it normally.
I haven't been able to figure out how to re-mount it then on the emulator (without rebooting it).
hints:
the adb command "remount" is unrelated: it's about /system
the emulator command is unrelated: it's only about starting the emulator
mounting the SD card on OSX without unmounting it from the emulator, causes horrible pain to the emulator