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:
UUID=6J8G2W6R9T /backup/external ntfs-3g defaults,nofail 0 0
Where the UUID is the disk ID, /backup/external is the path, ntfs-3g is the format and the last part are the options.
The nofail option is best used with the x-systemd.device-timeout option. This is because the default device timeout is 90 seconds, so a disconnected external device with only nofail will make your boot take 90 seconds longer, unless it is changed.
Setting the timeout to 0 will mean infinite timeout, so it defeats the purpose of nofail.