How to properly eject an external disk

Click on it's

⏏️

eject icon on the Finder Sidebar, or right click on the icon on the desktop and click on "Eject name_of _drive" from the menu.

click here ➜ Connect and use other storage devices with Mac - Apple Support

In addition to @Phil0124's information, I highly recommend waiting at least 15-20 seconds afterwards before physically disconnecting an external drive. I actually tend to wait 30 seconds or even as long as a full minute depending on amount of data I have written to the drive and the speed of the drive since it can sometimes take a while for macOS to flush the drive's write cache to the physical drive. Otherwise you may see "Disk not ejected properly" error message which can also indicate possible file system corruption & data loss on that drive. Years ago macOS would actually provide users a notice when it was safe to physically disconnect an external drive....I really miss that feature.

If your external drive has an activity LED, then you can watch it & when it stops blinking......wait a few more seconds to be sure, then physically disconnect it. Some drives when they are ejected may even turn off their power and/or activity LED....if you have one of these, then once the LED goes dark it is safe to immediately physically disconnect the drive.

If you get a "Disk not ejected properly" message, then make sure wait a bit longer next time before physically disconnecting the drive.