You should be able to set that anyway you want.
I have mine set to disc first and disk second.
That way I can just pop in a disc and it boots from that.
If there is no disc in the unit it just falls through and boots from the disk.

Since this all seems to have started with the ram movement grab a copy of memtest86 and let it run for a few errors allowed.

You don't want faulty ram...think dma... corrupting files on the disk, that just leads into a big mess.