OK. Here are some instructions from Gizmodo that will put the ISO onto the flash drive with a Mac... they won't make it bootable though... read on...

1. Open a Terminal (under Utilities)

2. Run diskutil list and determine the device node assigned to your flash media (e.g., /dev/disk2)

3. Run diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command; in the previous example, N would be 2)

4. Execute sudo dd if=/path/to/downloaded.iso of=/dev/diskN bs=1m (replace /path/to/downloaded.iso with the path where the image file is located; for example, ./windows7.iso)

5. Run diskutil eject /dev/diskN, and remove your flash media when the command completes (this can take a few hours on slower drives)

Now... after you read all that, on the Gizmodo page it says that if you want to make it bootable you should use a utility called Live USB helper they link to (which isn't there any more) and use a Mac mounting tool (which isn't there either!) to force the ISO to mount on the Mac so you can copy the files over. So... that article may prove useful, but probably not. I can find other copies of Live USB helper out there, but they are all Windows executables.

One method that will work is to swap the hard drive out of the PC and into the Mac. Put your Windows 7 installation disc into the Mac optical drive, and install Windows 7 on that drive. Once it is up and running (don't worry about drivers yet), follow these steps.

1. Run Command Prompt as administrator
2. Type to Command Prompt: %windir%\System32\Sysprep\Sysprep.exe, and hit Enter
3. In sysprep dialog that opens, choose “System Cleanup Action” as “Enter System Out-of-Box-Experience (OOBE)”, select “Generalize”, and select “Shutdown Options” as “Shutdown”. Click “OK”
4. Sysprep generalizes now your Windows 7 setup and shuts down your computer. Do not run any other programs during this phase!
5. Remove the drive from your Mac. Put it back into the PC.
6. Boot the PC from sysprep generalized hard disk. You will notice Windows booting as if it was the first boot after installation, installing default and updating registry. One or two reboots are needed, depending on your system specifications
7. When Windows finally boots up, you will need to enter all information as if this really was a new, fresh installation

All that is from here, but it applies in this situation a well, since you would be moving a Windows 7 installation from one computer to another. Considering that trying to make a Windows bootable USB stick with a Mac requires tools that either don't exist any more or are not being developed any more... this second method will most likely prove more dependable.

If Apple's supported method of using the createinstallmedia tool doesn't work, you can try other options below:

• https://support.apple.com/en-us/HT201372 - Create a bootable installer for OS X

With OS X El Capitan, Yosemite, or Mavericks, you can use a USB flash drive or other removable media as a startup disk from which to install OS X.

Really - try the above if you can, but as an alternative, here is a simple recipe for making a USB version of the OS X installer + associated tools. (Migration Assistant, Disk Utility, Network Utility, Terminal (that auto mounts your internal drive as needed), Firmware Utility and Password Reset Utility.

1) Use Disk Utility backup/restore to image whatever Snow Leopard DVD you prefer onto a HFS+ formatted GUID USB drive. I usually first create a IMG of the DVD, but it should work in one shot. Watch for restore errors - you may need the Disk Utility menu command Images -> Scan images for restore... before restoring an IMG to the USB drive.

2) bless the USB drive by selecting it in Startup Disk or using the command line. (Note - the manual page for bless may be moved, but the command still works on Mojave to bless an alternate boot. You can’t set this to be the default boot with System Integrity Protection but for this use, bless should still work.

It's not that hard to make a bootable image and far more secure to DIY. It also avoids the need of downloading commercial software from the internet. Happy media shifting all!

If you simply want a bootable thumb drive, just install OSX onto that volume - turning off all the extras you may not need. You can have your USB on one side and the USB install disk on the other to make your bootable drive ensuring all the Air drivers are on your USB bootable drive (which is often an issue when using another install's image - sometimes the drivers are not all there on older builds of the OS)

The best trick I have when you don't have enough space on USB to create the final updated version is to stage the bootable image by install first to a 25G partition on an external hard drive.

You can finish installing with lots of free space on the HS, run all the updates newer than your installer (10.6.8 for example), make an admin account, install the tools and apps you need. At the end of the process, you then thin out the things you don't need. DaisyDisk or WhatSize will show you all the large files and libraries. You can safely get rid of a lot of iLife and the associated Application Support. WhatSize has many options to strip out unneeded files, localizations and PPC code. Once you have slimmed down the bootable image on the HD partition, use the trick below to get it on the USB drive. Leopard was a tight fit in 8G but Snow Leopard is lighter and doesn't need to diet as much.

Edit: Here are terminal commands for rolling your own installer like the (read-only) one that ships with current MacBook Air. I have a Snow Leopard DVD as /dev/disk5 and my USB is /dev/disk6. The man pages for diskutil and asr are helpful if you run into little gotchas or have different needs than a basic one partition drive.

$diskutil list
/dev/disk5
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     Apple_partition_scheme                        *7.8 GB     disk5
   1:        Apple_partition_map                         30.7 KB    disk5s1
   2:         Apple_Driver_ATAPI                         1.0 GB     disk5s2
   3:                  Apple_HFS Mac OS X Install DVD    6.7 GB     disk5s3
/dev/disk6
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                                                   *8.5 GB     disk6
$diskutil partitionDisk disk6 GPT HFS+ MacUSB 100%
Started partitioning on disk6
Unmounting disk
Creating partition map
Waiting for disks to reappear
Formatting disk6s2 as Mac OS Extended with name MacUSB
Finished partitioning on disk6
/dev/disk6
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *8.5 GB     disk6
   1:                        EFI                         209.7 MB   disk6s1
   2:                  Apple_HFS MacUSB                  8.2 GB     disk6s2
$sudo asr restore --source /dev/disk5s3 --target /dev/disk6s2 --erase
    Validating target...done
    Validating source...done
    Erase contents of /dev/disk6s2 (/Volumes/MacUSB)? [ny]: y
    Validating sizes...done
    Restoring  ....10....20....30....40....50....60....70....80....90....100
    Verifying  ....10....20....30....40....50....60....70....80....90....100
    Remounting target volume...done
$sudo bless --mount /Volumes/Mac\ OS\ X\ Install\ DVD\ 1 --setBoot

Note: that the USB drive was renamed to be the same as the installer and the mount point in /Volumes has a space and a 1 tacked on the end.

Note: SIP ensures the —setBoot portion of the commands above will fail. Remove that portion and/or ignore any errors. You will need to select the USB another way than having bless perform a one step command. This is another reason to just use createinstallmedia from Apple to perform this.