Time Machine stopped backing up
I'm using Time Machine to back up to an external hard drive for a MacBook Pro Mid 2015 2.83Ghz 1TB running OS 10.14.6.
The last few times I've tried to back up, I've plugged in my external hard drive and let it sit overnight for 8 to 10 hours. I've changed my settings so that the computer doesn't go to sleep. When I return to my computer in the morning, after at least 8 hours. Time Machine still says "preparing backup."
I've tried restarting my computer. Still the same thing.
Any thoughts as to how to fix this issue?
MacBook Pro 15″, macOS 10.14