After you open Eagle, it shows prompt message "Error: UNKOWN: unknown error"

If the error prompt "Error: UNKOWN: unknown error" showed after the app is opened, this is usually related to hard-drive issues. If you are encountering the situation, please try troubleshoot steps as described below:

Potential Reason 1: Not enough power for the USB port

The USB 3.0 device was being connected on the USB 2.0 port
While most USB 3.0 devices are downward compatible, there is still a vast difference in the max. rated current between the USB 2.0 port (500mA) and the USB 3.0 port (900mA). This difference might cause insufficient power input, resulting in stability issues with the device.
Instability or insufficient powers with the USB hub
You might also encounter the issue if the connected USB hub(s) doe not have external power. Please try connecting the drive directly to the computer and check if the issue persists.

Potential Reason 2: The Third-Party drive tools are used

If you are using hard-drive tools like Tuxera NTFS / Paragon NTFS to read portable devices, the issue might be related to the tools. Please try copying all drive data to the local drive and see if the issue persists. (Note: Eagle team has run tests in regard of Tuxera NTFS / Paragon NTFS tools, the instability issue still happens regardless of the formats/tools.)

Potential Reason 3: Bad sector or damage in the hard drive

Bad Sector is also one of the rason being to the issue, the bad sector is often caused by incidents like "Incorrect ejection, instable power supply, fall damage". If the abovementioned troubleshooting steps did not help you out with the issue, you might want to try fixing the drive with system built-in disk check tool, please refer to the steps below:

Click "This PC" in the explorer, find the drive letter of the disk in question
Right-click on the dirve, select "Properties"
Click "Tools" Navigator, then, click the "Check" button in the Error checking to start the process
Check if the issue is solved after the process.