How to Fix Can't Transfer Files onto External Drive on a Mac

Do you know what format the external drive is.

If it is formatted as NTFS, macs cannot write to NTFS formatted drives,

there are third party paid apps which will let you do this.

Macs can read NTFS natively but not wrote, very silly if you ask me.

To determine how your external drive is formatted

highlight the external disk on your Desktop and press

Command- i to display the info panel under

general it will tell you the format.

Macs can read and write to FAT32, exFAT, MacOS Extended (Journaled) and APFS formatted disks.

Use the Disk Utility app in Applications> Utilities to reformat disks but this erases the contents

of the disks so be careful.

In addition to Eau Rouge's correct answer:

if the external drive is not formatted as NTFS, have a look again at the info panel (command-i) and scroll down to Sharing & permissions.

Is it 'read and write'? If not: unlock the padlock and make the change