The way i have read your question is:
Drive 1 - bootable
Drive 2 - bootable
two different operating systems...booting independently? so... If you boot from your 1st drive, can you see the contents of the second drive?
Or you have one OS drive (drive 1) and the second one is for data storage (drive 2)
and when you plug in drive 2 your system fails to boot.
In this case can you access the BIOS and see the drives? are the drivers powered up and can you feel/hear the disk spinning (n.b. this will not work for SSD)