If you ran the system file checker scan and didn't get any errors, it's possible that the problem lies with the external Seagate. Especially considering you've been able to use it in the past 6 months without problems.
You could try using a trial version of Hard Disk Sentinel to test the Seagate. It will try to find, test, diagnose and repair hard disk drive problems, report and display SSD and HDD health, performance degradations and failures. It will work on internal or external drives.
Hard Disk Sentinel Trial version - HDD health and temperature monitoring