Icy Dock MB994SK-1B multiple boot drives

I have just ordered an Icy Dock MB994SK-1B for my PC. I currently have 3 SSD's. 1 is currently my boot drive for Windows 10 and is working fine. I will transfer this one from it's current single removable tray to 1 bay of the Icy Dock.

I also have a previous version of my Windows 10 in which I cannot change the network adaptor settings due to VMware software locking the adaptor and no way to get rid of it. I want to put this into bay 2 of the Icy Dock. The question is what happens when the PC boots and sees two bootable drives? Which one will it boot from?
I want to erase the second SSD so that I can re-use it as a normal drive for important data as I no longer use the Windows copy on that drive.

The 3rd SSD has Server 2012 demo on it which I play around with when I get bored so just plug it in and boot from it when ever the fancy takes me.

Well it did not do what I expected. I had all three drives in the slots. Windows 10 Main in Port 0, Port 1 had nothing in it, Port 2 had the old Windows 10 bootable drive, and Port 3 had the Server 2012 R2 bootable demo on it.
I expected to be able to select which drive to boot from but only the DVD and one SSD showed up. All drives appeared on the SATA list along with the 2 3TB data drives.

I let it boot and it booted into the Server drive. Logged in and the other drives available and could see the data.

Looks like I will have to have only 1 bootable drive to ensure I get the correct one.

Made Port 0 none hot swappable and the other 3 ports hot swapped. Works a treat.

Now just have to save some more money for some more 2.5 drives.