Fails if multiple disks are already attached

[holotrek]

• Original error was:

$ ./prepare-iso.sh 
hdiutil: detach: only a single device name can be specified
Usage:	hdiutil detach [options] <devname>
	hdiutil detach -help

Mount the installer image
-----------------------------------------------------------
$ hdiutil attach /Applications/Install macOS Sierra.app/Contents/SharedSupport/InstallESD.dmg -noverify -nobrowse -mountpoint /Volumes/install_app
hdiutil: attach failed - Resource busy
Failed to mount the InstallESD.dmg from the instaler at Install macOS Sierra.app.  Exiting. (1)

• Found line that was attempting to call hdiutil detach, and ran all but the command to determine what arguments were being sent in:

$ hdiutil info | grep /dev/disk | grep partition | cut -f 1

• The output from this was:

/dev/disk1
/dev/disk2

... which explains why the error complained that "only a single device name can be specified".

• This was resolved by manually detaching each disk separately using the following commands, but just wanted to mention this in case there was a way you could iterate the results and call it for each line (not familiar enough with xargs to suggest a resolution).

$ hdiutil detach -force /dev/disk1
$ hdiutil detach -force /dev/disk2

[geerlingguy]

Looks reasonable to me, thanks for finding and fixing this!