The files that are opened by these actions were on an external HD,
I disconnected and then reconnected it, but in all the actions now, at the OPEN step it tells me file not found, having reconnected the external HD and not having changed the file path, how can I tell it to automatically find all the files?
I could do it by hand by changing the action option and reopening the file, but we're talking hundreds of files...
What OS are you using? I know that if you're on Windows, every time you plug in a hard drive, it will reassign a drive letter, unless you have manually assigned one. I had this issue with Lightroom. So I assigned each of my external drives a letter. This most likely is the same in that you need to assign a name to your drive. Look at your original action and see what the path is for the files, and change the drive letter to whatever is in your actions.
That being said, what happens if it changes the next time it mounts?
Perhaps a better option would be to convert the action to a script using xtools, then find/replace all copies of the path with the volume name. Unlike actions, script volume paths don't suddenly change!
Did you try to drag the file to your desktop and open through photoshop? If it allows you to open then it just lost its path to your external HD. Re-save to your external HD and it should work for the future.