This is one of the reasons I always erase external HD's and format them for macOS prior to use. Proprietary security and compatibility software can always become incompatible over time and 3rd party manufacturers don't always keep up to date, printer drivers being a classic example.
One of the links in the article from Support Seagate US regarding issues with upgrading to macOS Monterey actually recommends downloading a recovery software application from iBoysoft;
- Free download, install, and launch iBoysoft Data Recovery software on your Mac. For your information, first-time installation of this program requires you to allow Full Disk Access and enable system extensions on Mac.
Would I do this? No way! Its almost inevitable that they will charge you and I don't like giving 3rd party apps full disk access to my data. Here is one user review "
Online user documentation is very scarce. Also, I think it's a huge price from the normal version to the professional version, where only support for Windows Server is added. I also think that your refund policy is almost nil, if you read well there are no refunds." Ivan G. on G2.
Here is another review page worth checking out, especially re the cost for Mac version.
This is a review of iBoysoft Data Recovery, an all-in-one data recovery tool for Mac and Windows.
datarescuetools.com
Remember this is just data recovery, not fixing your problem. Your problem is the outdated propriety software.
Personally I would try plugging it into a Windows PC or Mac running older software and retrieving your data that way (free) then erase the drive at the top level (Media name) formatting it as FAT 32. That should make it visible to your Mac.
Connect the FreeAgent to any open USB port on your MacBook.
Open the Apple menu and click "Go" and then "Utilities." Double-click "Disk Utility."
Select the FreeAgent drive and open the Partition tab. Open the "Volume Scheme" drop-down menu and change the setting from "Current" to "1 Partition."
Click "Options," select "GUID Partition Table" and press "OK."
Change the "Format" setting to "Mac OS Extended" or APFS depending on your Operating System. Optionally, type a name for your disk in the "Name" text box.
Click "Apply" and then "Partition" to confirm that you want to erase the drive and reformat it. If Time Machine appears after the format, click "Cancel" unless you want to use the drive for dedicated system backups.