boot issues - long delay, then "gave up waiting for root device"

Solution 1:

Your latest dmesg messages look interesting.

I've seen similar press space type problems during booting on one laptop I owned.

The solution I found was to add to grub the noapic or the nolapic grub boot parameter.

There are a number of grub boot options you can try as per the community wiki:

https://help.ubuntu.com/community/BootOptions

Thus if standard workaround grub boot options such as noapic or acpi=off dont work for you, some of the other common options could work for you.

When testing grub boot option you could use my answer here to temporarily add the kernel options at boot time.

How do I set 'nomodeset' after I've already installed Ubuntu?

irqpoll:

from the wiki: Changes the way the kernel handles interrupt calls (set it to polling). Can be useful in case of hardware interrupt issues.

  irqpoll         [HW]
                        When an interrupt is not handled search all handlers
                        for it. Also check all handlers each timer
                        interrupt. Intended to get systems with badly broken
                        firmware running

source

Thus - and since you have a custom built computer - it is very likely to be one or more components that make up your computer. To discover which one will require a bit of detective work.

The easiest way would be just to simplify you computer to the basics - use a fixed keyboard, simple mouse, onboard graphics, onboard ethernet and nothing else. i.e. pull out all the PCI cards/USB components you are using. Then add each one back in one at a time.

If you find that you still need irqpoll on a simplified build - then you'll need to take a closer look at your motherboard. Look at the manufacturer site and see if there are any firmware updates - most probably you'll need to dos-boot to flash these.