Format a disk for Windows computers
You can format a disk or volume as a secondary volume to use with the Windows operating system you installed on your Mac using Boot Camp. You can also prepare an external disk for a Windows computer, by formatting it using two MS-DOS formats.
WARNING: Formatting a disk or volume erases all the files on it. Save any files you need to another volume before formatting.
Choose View > Show All Devices.
In the sidebar, select the disk or volume that you want to format to use with Windows computers.
Click the Erase button , click the Format pop-up menu, then choose one of the following:
MS-DOS (FAT): Choose if the size of the disk is 32 GB or less.
ExFAT: Choose if the size of the disk is over 32 GB.
Enter a name for the volume.
The maximum length for the volume name is eleven characters.
Click Erase, then click Done.
You can also partition a disk using the Master Boot Record partition layout, which is compatible with Windows.