If you have a non-existing (or unplugged) external drive in /etc/fstab, Linux will not boot until you either press S to skip or M for manual mount. This is a desired behavior, because the operating system does not know what is on that external drive and whether it should ignore the error. If the disk is just used for archiving, the system not booting can be a problem, especially if the disk is just temporarily removed.
To avoid this, you have to add nofail to the fstab, so if the disk becomes detached, the system will continue to boot.
Here's an example: