erase your external hard drive and format it to ExFAT.
you can find what format your external hard drive by:
disk utility > your external hard-drive name > below the icon and the name of your drive will say a format type like AFFS or Mac OS Extended (Journaled).
to change your external hard drive format follow these steps:
disk utility > your external hard drive name > right click > click erase ( this will get rid of everything on the external drive so make sure to back-up somewhere else ) > under format choose ExFAT > erase
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