I just replaced my Samsung 830 256gb SSD with a 1TB 850 Evo SSD (I am running OSX in Thunderbolt external) and my R/W speeds are significantly slower. I ran a speed test with drive in OWC USB 3 enclosure and I am getting the same speeds as when I am using the drive in Thunderbolt enclosure. The drive is supposed to get up to 540MB/s and I am getting between 350-390. I have enabled trim (El Capitan) and this makes no difference. Is there something wrong? Shouldn't I be getting better speeds with Thunderbolt?