苹果 mac osx 下吧iOS镜像写入u盘制作系统安装盘

How do I burn an ISO on a USB drive on Mac OS X?

Download the desired file
Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight)
Convert the .iso file to .img using the convert option of hdiutil (e.g., 

hdiutil
convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso

) Note: OS X tends to put the .dmg ending on the output file automatically. Remove the .dmg extension as necessary, 

mv
~/path/to/target.img{.dmg,}

Run 

diskutil
list

 to get the current list of devices
Insert your flash media
Run 

diskutil
list

 again and determine the device node assigned to your flash media (e.g. /dev/disk2)
Run 

diskutil
unmountDisk /dev/diskN

 (replace N with the disk number from the last command; in the previous example, N would be 2)
Execute 

sudo
dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m

 (replace /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img or ./ubuntu.dmg). Using /dev/rdisk instead of /dev/disk may be faster. If you see the error
dd: Invalid number '1m', you are using GNU dd. Use the same command but replace bs=1m with bs=1M. If you see the error dd: /dev/diskN: Resource busy, make sure the disk is not in use. Start the 'Disk Utility.app' and unmount (don't eject) the drive.
Run 

diskutil
eject /dev/diskN

 and remove your flash media when the command completes
以上来自；https://snipt.net/w00kie/how-do-i-burn-an-iso-on-a-usb-drive-on-mac-os-x/