The Court did overrule itself on "separate but equal," and regarding the internment, the decision was made based on military necessity, and while not morally palatable to us, it does appear to be constitutionally correct.
"It is said that we are dealing here with the case of imprisonment of a citizen in a concentration camp solely because of his ancestry, without evidence or inquiry concerning his loyalty and good disposition towards the United States. Our task would be simple, our duty clear, were this a case involving the imprisonment of a loyal citizen in a concentration camp because of racial prejudice. Regardless of the true nature of the assembly and relocation centers--and we deem it unjustifiable to call them concentration camps with all the ugly connotations that term implies--we are dealing specifically with nothing but an exclusion order. To cast this case into outlines of racial prejudice, without reference to the real military dangers which were presented, merely confuses the issue. Korematsu was not excluded from the Military Area because of hostility to him or his race. He was excluded because we are at war with the Japanese Empire, because the properly constituted military authorities feared an invasion of our West Coast and felt constrained to take proper security measures, because they decided that the military urgency of the situation demanded that all citizens of Japanese ancestry be segregated from the West Coast temporarily, and finally, because Congress, reposing its confidence in this time of war in our military leaders--as inevitably it must--determined that they should have the power to do just this. There was evidence of disloyalty on the part of some, the military authorities considered that the need for action was great, and time was short. We cannot-by availing ourselves of the calm perspective of hindsight-now say that at that time these actions were unjustified." [323 US 214, 223-224]
To get this back to the topic at hand, the rulings against secession have not been overturned despite the Court having ample opportunity to do so in several cases, and they fall in line with previous case law on the relations of states to the Federal government and to each other.
Regards,
Cash