Download DriveDX and run a report on the External Hard Disk, if any errors appear than the External Hard Disk is failing and needs replacing. You really have only described what you own and what you are sensing, we cannot diagnose what forever means to you only you can.
Also try restarting the computer in Safe Mode with the External Hard Disk cable disconnected then reconnect and restart normally and test the foreverness quotient. :-)
If it's still too long and you have no errors from the DriveDX report then please do the following:
It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.
When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again.
For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211 It is helpful to us if you download the report to your Desktop.
