So, you might have first formatted your External HDD on a Windows-based machine. Windows uses mostly NTFS formats when formatting External HDD (hard drive) or any hard disk for that matter. Note:- That the above problem can occur if you are trying to use an external hard drive that you normally use with your Windows computer. Within Disk Utility, select your drive from the list and click on “Change Format” and choose Mac OS Extended (Journaled) as the type of format that you want to proceed with.And to go Disk Utility in Applications and under Utilities.

This will wipe out the contents of your external hard drive. Fix unable to Copy MacBook data to External HDD