Thank you so much for writing this blog!
I spent two days trying different options and this one just worked in 5 minutes from start to finish (with a few adjustments) for the Crucial M500 MU05 firmware update.
Here is what I did (not sure if this made a difference or not but at least worked for me).
1. Before starting, I used disk utility and formatted the USB drive to "MSDOS (FAT)" and the "Master Boot Record" scheme
2. Once formatted, I unmounted the drive as well (just like you unmounted the SYSLINUX and rEFIt volumes) with
admins-Mac-mini:~ admin$ diskutil unmountDisk /dev/disk1
3. Used sudo to perform the dd command
admins-Mac-mini:~ admin$ sudo dd if=/dev/rdisk2 of=/dev/rdisk1 bs=1m
More information about formatting options