The reasons behind an unmountable external hard drive on a Mac are various, but the same result is that all your drive content is inaccessible. When the Seagate external hard drive can't be mounted, you can try the following fixes:
Check the connections including the USB cable, the USB port, etc. Make sure the involved parts are not defective and the connection is tight.
Disconnect unnecessary peripherals. If any peripheral is problematic, it may prevent the Seagate external drive from mounting normally on Mac.
Manually mount the drive via the Mount button. Just select the Seagate external drive in Disk Utility and click Mount.
Mount the APFS Seagate drive with the Terminal. Open the Terminal app and enter the command diskutil mountDisk /dev/identifier to mount the drive.
Repair the Seagate external drive with First Aid. A corrupted drive will fail to be mounted. You can try the First Aid feature of Disk Utility to check and repair disk errors.
If all of these methods fail, you may need to reformat the drive to make it usable again, but you can recover data from the unbootable drive before the reformatting. iBoysoft Data Recovery for Mac is a good choice for you.