First, I'd suggest attempting to mount the drive manually in Disk Utility. Here's how: Open Disk Utility -> Locate the drive on the left side -> Click on "Mount" in the toolbar. If the drive doesn't mount, you can attempt to run First Aid again.
If the above doesn't work, you might need to try using the Terminal to force mount the drive. Open Terminal (you can find it in Utilities), and type `diskutil list`. Find your Seagate drive's identifier (it'll be something like disk2s1), and then type `diskutil mountDisk force /dev/disk2s1` (replace disk2s1 with your drive's identifier).
Lastly, if your drive still isn't cooperating, you might want to consider using a third-party disk repair tool. There are several good ones on the market that can sometimes repair issues that Disk Utility can't.
However, I strongly recommend creating a disk image backup of the Seagate drive using Disk Utility before proceeding with any third-party tools. This is just to ensure that, in case anything goes wrong, you still have a copy of your data. You can follow the instructions on Apple's website for creating a disk image: Create a disk image using Disk Utility on Mac - Apple Support
Good luck!
References:
1. Disk Utility User Guide for Mac - Apple Support
2. Mount & Unmount Drives from the Command Line in Mac OS X - OSXDaily <<< A bit dated, but still valid.
3. Create a disk image using Disk Utility on Mac - Apple Support
4. Repair a storage device in Disk Utility on Mac - Apple Support
