external hard drive
why is my mac not able to back up to the external hard drive?
MacBook Pro
To use an external drive for backups, Disk Utility should "own" the drive.
That means you take the new drive aside in Disk Utility before you begin, select it my its immutable and complex manufacturer-given Device name, and ERASE it. If you are not seeing the manufacturer-name, use the little View Menu to "show all devices"
.