Backups to external hard drive unsuccessful every time.

So, this MacBook Pro is not backing up to an external hard drive and I've done everything I can think of to diagnose the problem but still haven't gotten anywhere.

I try and backup to the drive and it says:

"Time Machine couldn't complete the backup to "<> Backup".

"An error occurred while copying files. The problem may be temporary. If the problem persists, use Disk Utility to repair your backup dis.

Latest successful backup: 20 May 2018."

I brought it to Apple to see what they had to say and they were useless and offered no support whatsoever.

I have ran First Aid on the Disk through Disk Utility and I'm still stuck. I have another Backup drive that also does the same thing... There are no available updates or drivers for either of the drives. It used to work but isn't now. I've tried all the USB ports on the machine. So, I'm a bit stuck.

The two drives are:

a Seagate Backup Plus Slim (main disk)

a G Drive mobile USB.

As I said, both used to work but now it just doesn't. Preparing backup takes a very long time too before it starts the process but then comes up with the error soon after.

Thanks,

Sam.