When I attempt to back up to an external hard drive the following occurs;

"File History doesn't recognise this drive"

The external drive is there and I have successfully copied files to it.

This has occurred since my unsuccessful attempt with Windows 10 after which I reverted to 8.1

Can any one help me please

You could try the following method.

Click Start -> Run --> Type services.msc and press the Enter button.

Find the service "Windows Search" and enable it.

Hi Joannie,

I recommend you refer to the following similar thread and it may give you some hints:

http://answers.microsoft.com/en-us/windows/forum/windows_8-files/file-history-unknown-error-file-history-doesnt/3c24ecf2-751c-484b-a563-a600af435cb1

Best regards,