My external hard drive has unmounted itself and won't remount ?

Did you physically disconnect the drive after it became unmounted, then reconnect it to the computer?

Have you tried rebooting the computer? Or tried accessing the drive while booted into Safe Mode?

How is this external drive being used? Time Machine backup drive, or a backup drive using a third party app, or just a simple data drive?

You omitted a lot of critical information such as:

• Exact model of the computer and version of macOS being used. You can get this information by clicking the Apple menu & selecting "About This Mac".
• Exact make & model of the external drive....a direct link to the product page is best
• File system being used on the external drive
• Age of the drive
• How the drive is connected to the computer

FYI, anti-virus apps, cleaning/optimizer apps, and third party security software can easily cause problems. If you are using any of these types of apps, then uninstall them by following the developers' instructions since these types of apps are not needed on a Mac.

Marty_Mac wrote:

I was using it last night but has unmounted itself and won't re-mount. Have tried first aid on my external drive but, it keeps saying it is unmounted. Does anyone have any ideas why my MacBook is not recognising my External Hard Drive or, why it won't mount ?

Please can anyone help me ?

Marty

All drives will fail in due time. Not if, but when...

I would try a different external drive and compare you results.

All new drives need to be formatted: erase/reformat/initialized as new

Erase and reformat a storage device in Disk Utility on Mac

Erase and reformat a storage device in Disk Utility on Mac - Apple Support