Two possibilities. One is the hard drive has a hardware issue. The other possibility is the file system has a problem which First Aid is unable to fix.
Since you are using the HFS+ file system on the drive, you can try running the paid third party app Disk Warrior to attempt repairs on the file system. However, if Disk Warrior detects a hardware issue with the drive, then no repairs will be performed. Disk Warrior is very good at repairing the HFS+ file system as long as the drive is healthy.
You can attempt to check the health of the hard drive by running DriveDx and posting the complete text report here using the "Additional Text" icon which looks like a piece of paper. You will need to install a special USB driver in order to attempt to access the health information of the external drive. However, even with the special USB driver, there are no guarantees that the USB chipset used by the external drive (or adapters) will allow the necessary communication to access the drive's health information.
It is always risky attempting repairs to a file system even though I have never had a problem with Disk Warrior (there is always a first time). I hope you have a good backup of the data on the drive.
You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.
Edit: If the hard drive is failing, then you need to be extremely careful since the more you have the drive powered on, the more likely the failure will get worse. Also, the more you attempt to access the data or attempt to repair the file system, the more likely the failure will get worse (sometimes very quickly) where even a professional data recovery service may be unable to recover any data. macOS and most apps including data recovery apps are not designed to handle a failing hard drive producing lots of errors. In fact macOS and most apps will get stuck trying to deal with the errors making the problem worse. If the hard drive is failing you may only get one chance at recovering data from a failing drive so choose wisely.
