how to backup external hard drive time machine after setup

The drive should mount on your desktop.

Wait 10 seconds, then plug the drive back in and turn the power to the drive back on.

Once the drive is ejected, you can turn it off or disconnect its cable.

Select the Time Machine drive from the Disk Utility sidebar, and then click the Unmount button in the toolbar.

If your Time Machine drive isn't mounted on your desktop, launch Disk Utility, located at /Applications/Utilities.If your Time Machine drive is mounted on your desktop, right-click the drive and select Eject 'drivename' from the pop-up menu.While I can't tell you the reason, I can tell you that this is by far the most common solution for the 'backup volume is read only' error. If you're using an external drive connected to your Mac via USB, FireWire, or Thunderbolt, you can try ejecting the drive from your Mac and then reconnecting the drive or restarting your Mac.

Launch System Preferences, and select the Time Machine preference pane. But don't head off and try to reset the permissions because it won't do you any good. The cause of the error message depends on a few factors, but in all cases, your Mac thinks the drive has had its permissions changed to be read only. The bad news is that you can't backup any new data to your Time Machine drive until you get this problem fixed. The good news is that your backup files are probably all in good working order and no backup data has been lost. One of the common problems you may come across is Time Machine being unable to access the backup disk. But like all backup applications, Time Machine is subject to errors and problems that can creep in and cause you to worry about your backups. Time Machine is an easy-to-use backup system with a nice collection of features that make it the go-to backup system for most Mac users. Fixing Time Machine Errors - The Backup Volume is Read Only

How to backup external hard drive time machine after setup