Dell Inspiron 15-7559, Will only first boot M.2 Slot over 2nd HDD

Hello,

Got the Dell Inspiron 15-7559

Standard with 100 gb 2,5 inch HDD.

Inserted a Kingston 2.5 inch HDD, works fine.

Now days later I also inserted a M.2. Sata SSD. 

Both Disks work.

But when selectign legacy boot and first selecting the second HDD it does not boot up the 2nd HDD. 

It gives an error message saying boot.msgr is missing.

Tried :

- Legacy mode, Secure mode on and off
- Turning back to Uefi mode with secure mode on and off

- Legacy mode and all kidns or boot orders.

When I press F12 at startup and go to boot selection and select 2nd HDD, there is no problem, it boots up my 2.5 inch ssd no problem.

Now I installed windows also on my M.2. Sata SSD and this one boots up fine in legacy mode. No issues at all. 
But also now when setting 2nd HDD first it still boots up the m.2. And once again manually selecting the 2nd HDD at startup works fine.

I would like to use the 2.5inch ssd as boot device and the M.2. sata as storage.I have my reasons for this lol. :) 

Can anybody supply me with some advice/help?

Thanks in advance!