Question HDD with OS on it disappeared

[mbwells2673]

I installed an SSD drive on my wife's PC. Install went fine and she was able to use both the old HDD (that still had operating system on it) and the new SSD. When she started migrating non-OS files to the new SSD, something went wrong and the machine crashed. When she tried to reboot, it gave an error message indication no boot device. I went into the BIOS and the old HDD with the OS on it does not show up now. Only the new SSD shows up in BIOS which does not have an OS on it.

I pulled the old HDD out and connected it to a drive wire on my PC and the old HDD seems to be fully intact and functioning.

What do i need to do to get the old HDD show up in the BIOS again?

 

[Lutfij]

Welcome to the forums, newcomer!

We will need a little more information. Please list your specs like so:
CPU:
Motherboard:
Ram:
SSD/HDD:
GPU:
PSU:
Chassis:
OS:

You should've ideally backed up all critical content onto a removable device/storage prior to installing the OS on the SSD. If anything in it's current state, move the data to avoid loosing them when the older OS installation goes belly up.

Please state what the OS on both drives are.