Since my wife got a new iMac I took her old mid-2011 21.5" iMac that has a Samsung 840 Pro SSD installed. After erasing the drive, I installed a fresh copy of Sierra. After loading a few applications, I decided to run Blackmagic disk speed test. The read speeds were ok at 350-400 mbps. The write speeds however were abysmal at 20-25 mbps. The drive has the latest firmware running. Any ideas where the problem may be and how to fix it. Since it's an iMac, removing the drive for testing in other systems is not realistic. I have the same drive in my late-2011 MBP and I get write speeds of 240 mbps and read of 480 mbps. Same firmware also.