By default, macOS formats USB storage devices (such as flash drives) with a file format that is not recognized by Windows. In order to use a USB storage device in Windows, it will need to be "formatted" with a file system that both operating systems recognize. NOTE: This process will erase all data on the storage device, so be sure to copy any necessary files to the computer first.
- Connect your USB storage device to a Mac computer.
- Open Disk Utility (Applications > Utilities > Disk Utility or do a Spotlight search).
- Select the device from the list in the External section on the left-hand side.
- Click the "Erase" button.
- Select "ExFAT" as the format. The scheme can remain as "GUID Partition Map".
- Click "Erase" and follow all remaining prompts to continue. If macOS then prompts to use the storage device as a Time Machine backup, decline.