You have two disk drives with grub only installed on one of them. Most likely, you have an old, broken copy of grub installed in the other and that is what the system booted. Run dpkg-reconfigure grub-pc, and install grub to both drives so that either one should work.
You have two disk drives with grub only installed on one of them. Most likely, you have an old, broken copy of grub installed in the other and that is what the system booted. Run dpkg-reconfigure grub-pc, and install grub to both drives so that either one should work.