My Seagate is locked. I can’t add or delete files. It is used mostly on Mac.
In addition to the above answers what you can try, if available:
select the external hard drive
click File > Info (cmd + i)
At the bottom of the Information window expand the Sharing and Permission information. Unlock the setting by clicking on the lock icon, then check the "ignore ownership on this volume" box.
How to Fix a Read Only External Hard Drive on a Mac
It is likely the drive is mounted as read-only. Open a terminal and type the command 'egrep " ro,|,ro " /proc/mounts' skipping the single quotes at the beginning and end, keeping the double quotes and single spaces within the quotes. This will return a list of all read-only mounted filesystems. If the drive in question is in the returned list, use the mount command to remount it as read-write (i.e., mount -o remount,rw /mount_point).