Seagate hard drive

I was afraid of that. Had the same issue with a brand new external hard disk back then. Was also formatted as NTFS by default. MacOS can read from NTFS partitions. But can't write to it unless you use 3rd party software like NTFS for Mac. NTFS is what Windows uses by default. So a Windows computer won't have any issues.

What you need to do is open up Disk Utility and select the drive on the left side bar. Then click on the Partition button.

This will open this window

Here you can set the kind of partition you would like. MacOS Extended (Journaled) is generally the best option for an external Mac hard disk. Make sure you have a backup of the files on the hard disk. Because after this step that data will be gone. Then select Apply.

If I'm not mistaken MacOS will now format the hard disk and it will be available once it finishes :)

How is the drive formatted? If it's NTFS then Catalina can't write to it. If that's the case you need to reformat is as either APFS or MacOS Extended (Journaled).

Check with Apple's Disk Utility. The example below is my own external SSD which is connected to my MacBook Air.

Mine is formatted as MacOS Extended (Journaled). Which partition type is yours being listed as?