I installed Windows 10 to my new SSD while my old hard drive was disconnected. I did this because I wanted Windows to install everything (Bootloader, Rescue partition, etc) to the new SSD.
Afterwards I connected the old drive again and it didn't show up at all in Windows (not in diskpart, not in Disk Management, not in Windows Explorer). However, in BIOS I can see the drive with my old installation. I can also see it when I boot into the Windows Installer again (Install > ... > Select drive to install Windows - it shows up there with all partitions along with my SSD).
I need my old drive as there were additional partitions for my data which I now can not access anymore.
How do I access my old hard drive after booting the new OS on the new drive?