after you write the image you've got to resize the "mapping" partition
os x can't resize it in disk utility so you have to use terminal
taken from this thread on macrumors:http://forums.macrumors.com/threads/edi ... ap.872009/
1) Do a Get Info in Disk Utility on the drive you want to modify to obtain its Disk Identifier, which will probably be in the form diskn, where n is a small single digit.
2) Also using Disk Utility, back up the volume (partition) you are going to modify (if it has anything you want to save on it) by creating an image file of said volume. If there are any other files or volumes on the disk that you really, really don't want to risk losing, back those up also, just in case!
3) Open Terminal and type
fdisk -e /dev/<disk identifier>
where '<disk identifier>' is the name you obtained in step 1. Ignore the message:
fdisk: could not open MBR file [...]
that you will probably then see.
to see a list of partitions on the disk. Then type
where <n> is the number (from 1 to 4) of the partition you want to edit, which had better be the last one on the list of partitions unless you are knowledgeable enough not to have needed these instructions in the first place. You will be given a sequence of prompts that you can respond to by pressing <RETURN> to choose the default. However, if you don't want to use up all the space at the end of the disk for the partition you are expanding, you can respond to the 'Partition size' prompt with a smaller number. In addition, if the default response to the 'Partition offset' prompt is a lower number than the existing start sector number for that partition, you may want to respond to the prompt with the latter number, so that the starting location of the volume remains unchanged.
5) After you do this, type 'p' again to make sure that you haven't changed any other partitions, then type 'w' to write the new partition table to the disk. Despite any warnings you will see before confirming this, writing the edited partition table to disk will not change anything except the partition (volume) whose entry you edited, and, in most cases, won't change any data even on the edited volume, although the OS may, when that volume is mounted, treat it as unreadable or problematic. In that case, you may want to restore the contents of that volume, or some part of them, from the backup you made in step 2.
I just did this last night for a 32gb disk for my minimad and it worked great.