Laptop cant boot from ssd when hdd is coectd

I've just installed a new ssd on my laptop instead of the previous hdd.
When only the ssd is coected - everything works great (I've copied the os and data from the hdd to the sdd). But when I also coect the hdd, the laptop doesn't succeed booting from the ssd.

• I can boot from ssd if it's the only drive coected.
• The laptop did succeed in booting from the old hdd when both are coected.
• I've tried changing the boot order and also deleted the hdd from boot options - still not booting.

I thought that maybe the fact that I still have partitions and os on my hdd interrupts the ssd from booting, but it's weird cause it shouldn't affect anything (from what I know) if the hdd is not even a boot option.

Details:

• I've copied everything from the hdd to the ssd with Samsung Data Migration utility.
• It's an Asus laptop (n550jv) if that matters.
• The ssd is coected where the hdd was.
• The hdd is coected with hdd caddy instead of the cd-rom.

Maybe some boot configuration I can set? Or I'll have to format my hdd and start using it fresh?

Thanks!