When you plug an external hard drive plugged into your Mac, you might prefer to set the permissions so that the drive is only readable, to prevent people from adding files to it, but then later realise you need to make the drive writeable. For example, you have an external hard drive whose contents you intend to protect from being overwritten, or you want to provide read-only access to a portable drive that contains files you want to share. If you determine that you now need to write files to the drive, you can easily change its permissions.
Click the icon of the external hard drive, mounted on the Mac's desktop, to select it.
Click "File" from the Finder menu, and then click "Get Info." The external drive's information window opens.
Click the drive's of a lock icon to unlock it, and type your Mac's user account password if prompted.
Click the name of the user for whom you want to enable write privileges under the "Name" column in the "Sharing and Permissions" panel in the Info window.
Click the "Privilege" pull-down menu, and then click "Read and Write" to make the external hard drive both readable and writeable, or click "Write Only" if you want to turn it into a "Drop Box," which the user can only write files to, but not read files.
Click the lock icon to lock it and save the permissions changes.
Click the red circle at the upper left of the Info window to close it.