"Time Machine couldn't complete the backup to [external 1 T hard disk; Seagate]

Hello Anastasius1,

Let's start off by checking the helpful steps in this article which can help with this issue: If you can't back up or restore your Mac using Time Machine - Apple Support.

"If Time Machine says that it can't use your backup disk to back up or restore your files, try these solutions.

• Make sure that you're using a backup disk supported by Time Machine.
• Unless you're using a network backup disk, make sure that your backup disk is connected directly to your Mac, not through a USB hub or other device.
• If your backup disk is connected directly to your Mac, use Disk Utility to repair the disk.
• If you're using a network backup disk, make sure that the network connection is good and your Mac is connected to the same network as the backup disk. If you completed a previous backup successfully, verify that backup: Press and hold the Option key while choosing Verify Backups from the Time Machine menu in the menu bar.
• Make sure that the firmware of your backup disk is up to date. Check with the manufacturer of the backup disk for details.
• If no other solutions work, start over: Erase your backup disk, then use Time Machine to select it as your backup disk again."

In addition, the following page offers some great troubleshooting for these types of issues: Time Machine troubleshooting on Mac - Apple Support.

Let us know if we can help with anything else at all or if this doesn't settle your issue so we can look into this further with you. Thanks and have a fabulous day!