For whatever reason, though at this point I feel that the BIOS is suspect, my U400 can't boot while a drive caddy I bought to replace the ODD is connected. Going into the BIOS settings (which I can access fine even with the caddy connected) the hard drive mounted in the caddy is properly identified, as are my two other drives. However, when I actually try to boot off of one of the drives or even from USB (to run the WindowsRE, for example) it just hangs. It initially restarts the computer, but on the second attempt just hangs without any change at the Lenovo boot screen, minus the keyboard options. I've gone over the wiring in the caddy and the adapter the laptop uses to connect to the motherboard, and it all is fine. I've also checked out the drive in the caddy, and ensured that it was a simple logical partition that the BIOS or BCD couldn't confuse as vital for boot.
The BIOS version is 57CN30WW and changing the SATA Controller's working mode from AHCI to compatible doesn't change anything. Additionally, removing one or both of the other drives does not change the problem.
I looked online but couldn't find similar problems that were relevant as far as the issues and solutions went, so if someone here could give me a bit of guidance (I have done virtually nothing with laptop BIOSes) I would very much appreciate it.