I recently replaced the Superdrive in my Mid-2012 MacBook Pro with a drive caddy containing the original 500gb hard drive, and replaced THAT drive with a 256gb SSD. This setup has been working for some time, and I even installed Boot Camp on the solid state drive; Windows and Mac coexisted peacefully 
.
I decided recently to give the SSD entirely to Mac and give Windows the traditional hard drive.
I'd read that Boot Camp allows installation onto a separate INTERNAL drive, with the biggest example being on Mac Pro's. The utility even gave me the option of doing so, allowing me to set up boot camp on the secondary drive. I removed the primary drive as the Boot Camp manual instructed and installed Windows on the hard drive with no issue (I formatted the volume labelled "BOOTCAMP" during Windows setup, as instructed; failure to do this apparently causes the "no bootable device error" for other people, so I did it to ensure that my problem was not confused for that one). I rebooted and installed the Boot Camp drivers; again, no issue. When the time came to reconnect the first hard drive (again, as instructed by the manual), Mac booted just fine. However, attempting to boot to the Windows partition gives me the "no bootable device detected" error.
I disconnected the first drive again, and Windows booted without issue.
How can I fix this error and use Windows under Boot Camp on a separate internal hard drive, as Apple's manual seems to suggest I can?
.I decided recently to give the SSD entirely to Mac and give Windows the traditional hard drive.
I'd read that Boot Camp allows installation onto a separate INTERNAL drive, with the biggest example being on Mac Pro's. The utility even gave me the option of doing so, allowing me to set up boot camp on the secondary drive. I removed the primary drive as the Boot Camp manual instructed and installed Windows on the hard drive with no issue (I formatted the volume labelled "BOOTCAMP" during Windows setup, as instructed; failure to do this apparently causes the "no bootable device error" for other people, so I did it to ensure that my problem was not confused for that one). I rebooted and installed the Boot Camp drivers; again, no issue. When the time came to reconnect the first hard drive (again, as instructed by the manual), Mac booted just fine. However, attempting to boot to the Windows partition gives me the "no bootable device detected" error.
I disconnected the first drive again, and Windows booted without issue.
How can I fix this error and use Windows under Boot Camp on a separate internal hard drive, as Apple's manual seems to suggest I can?