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Select all the individual files/folders in the Finder window and Copy (or Drag) them to the USB. Mount the ISO by right-clicking on the ISO and choosing Open.
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Note: There is some confusion whether the entire ISO is copied as a single file to a FAT-formatted device which does not support files larger 4GB.
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You will also need to add the Bootcamp drivers to the USB as noted in step 11 of section Follow these steps if you downloaded version 5 of Windows Support Software in Install Windows 7 on your Mac using Boot Camp - Apple Support. Bootcamp 4/5 drivers from Apple to the root of the USB for W7/W8.1, as appropriate.These can be downloaded using BC Assistant -> Action -> Download. Bootcamp 6 drivers from Apple to the root of the USB for W10.Note: If you use W10 1903 or later ISOs from Microsoft, the install.wim is larger than 4GiB, and will not fit on a FAT32 USB.
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