Ah, yeah, your right, sorry--12 hour car drive, saw something that looked familiar and jumped to a conclusion.

Looking over the difference between those two boot logs here are some things that could be causing it:

1) scsi0 gets initialized on the failed boot, and sda4 is never mounted with the proper mount options (it attempts to mount with no options, and then it looks like on your successful boot it did a unexpected shutdown recovery of sda4 and it worked).   I don't know what you keep on this partition, but if it contains shared libs, then that would definitely cause a hang.  scsi0 might be a system-reserved recovery or efi shell for windows 8 fast boot that might be interfering with your boot process.  That would be a BIOS setting of disabling fast-boot to go about troubleshooting.

2) Dell Systems Manager Bus Driver is loaded on the successful boot, it doesn't seem to be on the failed boot, so could be hanging.  This handles power management routines on Dell computers, and can interrupt the boot process.




As a side note, cups and samba are failing to initialize on boot so you might want to disable those.

bumblebeed doesn't even attempt to initialize (this is a daemon that takes all your graphics card frame buffers, puts them in a single buffer and outputs it to one monitor, so it lets you have multiple GPUs on one box) on the failed boot.  It fails on the successful boot, but it's never even called on the failed so this is probably not the issue--if you disable it you'll likely see better boot times.

I don't know exactly what could cause this, but I'd reseat all the sata cables inside my computer and disable fast boot to start troubleshooting the problem.