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...
Open a Terminal (under Utilities)
diskutil list and determine the device node assigned to your flash media (e.g., /dev/disk2)
diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command; in the previous example, N would be 2)
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)
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.
- Run Command Prompt as administrator
- Type to Command Prompt:
%windir%\System32\Sysprep\Sysprep.exe, and hit Enter
- 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”
- Sysprep generalizes now your Windows 7 setup and shuts down your computer.
Do not run any other programs during this phase!
- Remove the drive from your Mac. Put it back into the PC.
- 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
- 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.