techsupport question

Hi, I need some assistance with dual-booting Windows 10 and Ubuntu.

I used to have two partitions on a 500GB SSD (/dev/sda), until I decided to move the Ubuntu-partition to a new SSD (/dev/sdb) for extra space. This worked well, and I could boot into the moved Ubuntu, using the GRUB menu.
A few days ago, I booted into Windows to get some documents, but as I tried to reboot into Ubuntu, I got errors (something like "could not access system ressources"). It seems like Windows decided to overwrite something.
I still have access to the "old" Ubuntu, which I luckily did not delete from /dev/sda yet. I have tried adding /dev/sdb to the GRUB menu, using boot-repair, but it only shows me /dev/sda (Windows and old Ubuntu).
I have very limited experience with boot-loaders, and thought it would be a good idea to ask for help, before messing something up, to the point of not being able to boot at all...

Any help/pointers would be greatly appreciated, let me know if I need to elaborate on anything.