viriatovigo; Hi ! Welcome to the forum.
What apparently is not happening is a bootloader on the second drive, to boot up any operating system.
Here is one solution:
You will need a LiveCD/USB. Boot into the Live Environment and go to: ctl+alt+t ->Terminal;
to verify what/where is on the disks. The following is "assuming" the second disk is ubuntu AND seen as "sdb" with a series of numbers appended for the partition numbers.
Mount your OS partition:
Next, install GRUB2 (ubuntu's boot loader):
sudo mount /dev/sdb1 /mnt
(Don't put a partition number in this command, just the HDD location a, b, etc.)
sudo grub-install --root-directory=/mnt /dev/sdb
It is also possible may need to edit the file system table file to automount, will see.
It is your system, up to you how you want to boot. My suggestion for now:
Set in bios to boot the 2nd hard drive(ubuntu) as the first boot priority.
Boot ubuntu and log in;
ctl+alt+t -> terminal:
will rebuild the bootloader config files and pick up the windows system and chainload it --giving the option to boot up windows from ubuntu's grub menu.
We have not touched disk 1 (windows) and windows boot loader remains. If you need/want to only boot windows at some point/time, just reset the boot priority to the 1st hard drive in bios.