Flashing folder icon appeared once after changing Macbook hard drive
This could have been a situation where the PRAM somehow lost the startup disk settings. If you reboot holding Command+Option+P+R and wait for the startup chime (or two) that will reset the PRAM and might fix the issue.

What you did was exactly right, however, as there might have been disk corruption or some other sort of disk problem. Unfortunately disk utility isn't as verbose as many would wish. Other disk utilities like Diskwarrior, TechTool Pro or Drive Genius might give you a more detailed report And even a freeware SMART utility might give you info on the health of the drive that Disk Utility won't report.

In other words you might be fine and this was a random occurrence (Murphy was a computer engineer...) or it could be the sign of an impending failure.

I would check it out further, and back up my data.