mmm...only need to know if it can be mounted and is readable.
So to see it can be mounted and is accessible. Then you would expect that all conditions are met to let it boot.
Typically at boot the blue led will blink and after disk has started and detected it should blink with disk activity for a few minutes while loading the firmware from disk.
What happens if you boot with the blue led.
If it again fails then take out the disk and connect it again to Ubuntu and mount sdb7
If the kernel was loaded successful and started loading the firmware then a file with name boot.log should be present in sdb7 root directory. If that is the case we can look at the content why it stops booting. If it is not there then probably loading of the kernel fails. Very unlikely unless the file you used is corrupt