external hard drive doesn't stay mounted

Hello Everyone,

I have a Mac Mini, M1 2020, running Mac OS 14.1.1 and using time machine to back up the internal hard drive to a Western Digital external drive that is plugged into an external USB hub. Time machine will back up to it for a week or two then won't. If I unplug it then replug it back into the computer, time machine will back up to it again. But only for a week or two and this cycle repeats. How can I keep the external Western Digital hard drive mounted at all times???? All this time the Mac see it but yet won't back up to it.

Even tried using disk utility, first aid, and it can see the hard drive but received an error that it could not "unmount" the volume for repair. What does this mean????? This is all very confusing, can someone offer any ideas?

Also, this issue has been with me for many years with two different computers and two different external HD's.

Thanks for any assistance you can give.

Be well,

Lou