Should I set up an appointment to remove the Seagate Backup Plus? And how can I check to make sure we are still getting backed up by Time Machine?
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While you don't necessarily need an appointment to deal with your backup, I can't completely and clearly answer your questions without more information. First some vocabulary:

Seagate is the hard drive you may be using to backup your computer. The Seagate drive is being used by Time Machine.
Time machine is the software that runs on your computer that backs everything up periodically. Time machine needs an external hard drive in order to do its job.

The seagate drive and time machine are co-dependent unless you have a different drive. To make things sometimes more complicated, it is possible to use the software that came with the seagate drive (western digital does this also) to backup. I do not recommend anyone use that software. I prefer time machine.

So let me know if this helps clear things up and/or if you feel you need assistance you can schedule a remote session and I'm sure we could take care of any additional issues.