What is File system check exit code is 8 when performing an FSCK?
When I do a first aid on an external drive, I get an exit code of 8 when it runs FSCK. What does this mean
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It would have helped if you provided more details such as the file system being checked. If First Aid is unable to repair the file system (APFS), then it means you need to erase the drive and restore the data from a backup. If the drive is using HFS+, then you can try running Disk Warrior (third party paid app) to repair the HFS+ file system. If Disk Warrior is unable to repair an HFS+ file system (rare, but it can happen especially is a physical drive failure), then again you will need to erase the drive & restore from a backup. Unfortunately there are no third party repair utilities for the APFS file system since Apple has not yet released the necessary APFS documentation.
You also need to consider what may have caused file system corruption. A failing drive is always a likely reason. You can attempt to check the health of the external drive by using DriveDx. You will need to install a special USB driver in order to attempt to access the health information on a USB drive. However, even with the special USB driver some USB controllers in the peripherals don't always allow the necessary communication to access the drive's health information. Post the complete text report here using the "Additional Text" icon which looks like a piece of paper.
FYI, a quick online search shows "exit code 8" means the file system is not repairable by fsck. If you want more specifics, then you could check the source code for what produces "exit code 8". Most of these cryptic error codes are not publicly listed (or at least not easily located).