Back up to Seagate hard disk

I use a Seagate external drive for backing up with Time Machine. As purchased, it is formatted for use with Windows computers and, as Allan Eckert suggests, it needs to be formatted for Mac use. Use Disk Utilities to format the drive - this link shows you exactly how https://www.macworld.co.uk/how-to/mac/how-format-drive-for-mac-3541758/

Depending on how large your drive is, you might want to consider partitioning it with, say, 75% for Time Machine backups, and the remainder for storing duplicates of particular files or photos.

Make sure that the hard drive is connected to your Mac. Click on the Apple symbol at the top left of the screen, select System Preferences and then select Time Machine. When Time Machine preferences popup appears, select Choose Disk and you should see your hard drive listed. Then select that, which will connect the drive to Time Machine. There will also be a small Time Machine symbol near the top right of the screen, which you can also use to get Time Machine preferences and other options.

How is the Seagate hard disk formatted?