Not using a Mac, but did you unmount the SD card first (as far as I remember, this is needed on a Mac). Try the following:
1.) Become root:
2.) Plug in your SD card and then use the following command to see which /dev/diskN node it's located on:
3.) Unmount the disk where "N" is the number of the disk taken from the above command:
diskutil unmountDisk /dev/diskN
If the above command was successful, you will see:
Unmount of all volumes on diskN was successful
4.) Use the 'dd' command to copy the image file to the entire disk:
dd if=image.img of=/dev/diskN
Keep in mind that this process can take quite some time (sometimes even 30 minutes), depending on the speed of your SD card etc. Leave it be just in case, it should either quit itself with an error, or finish with all ok...
Johan Dams, Genesi USA Inc.
Director, Software Engineering
Yep, I have a blog... PurpleAlienPlanet