I have a server HP Proliant DL120 G6, with an installed and working Ubuntu 14.04. The partitioning is reported below.

Now, if I move the disks on a very similar HP Proliant DL120 G5, in the same order, they don't even boot: no error, nothing, as if the disks are empty. I tried switching them, no luck.

This baffles me. How is that possible?

Details

The disks can be seen by the new server, and I don't get a "boot error" from BIOS. I just keep seeing the cursor blinking at the top left of the screen.

None of the servers has additional RAID controllers.

The destination server is actually booting and working with its own disks: my original plan was to replace the existing disks and to save time I installed the system on a different system.

Booting from a LiveCD, I can confirm everything is fine on the disks: partitions are there, the RAID array are ok.

Partitioning

/dev/sda1 + /dev/sdb1 = /dev/md0 = /boot
/dev/sda2 = swap
/dev/sdb2 = swap
/dev/sda3 + /dev/sdb3 = /dev/md1 = /

Note

I know how to fix the problem with a boot repair disk, or I can even reinstall for that matter. I am just very interested to know what is the possible cause for this issue.