Seagate (Samsung) external drive USB 3.0 not mounting on High Sierra
Looks like hardware failure to me. It might not be the drive itself. Could be a bad connection, or bad power supply. You haven't given any specific details on how you have this drive hooked up to your computer. Nor whether it's a 2.5, or 3.5" drive. So a definitive answer isn't possible. It it's read/write, tracks/sectors failing, smartmon will indicate to your BIOS/OS that the drive is failing, and needs to be replaced. You should perform a fresh boot and immediately look at your logs to see whether smartmon is reporting failures. But do check all your connections before powering up the computer &amp;&amp; hard drive.

Good luck!