I installed an Intel SSD (INTEL SSDSC2BP480G4) and replaced original hard disk with HGST HUS726060ALE610 on my iMac (27-inch, Mid 2010). Since I use SSD for system and the HGST hard disk for data backup. I did not pay attention to performance of HGST hard disk before. Recently, I want to build Android's AOSP on my iMAC. It requires to download over 100G data from repo to hard disk. I noticed that the speed was very slow. Then I ran black magic speed test on HGST hard disk and I found that the read/write speed was only 20MB/s. I double checked it with dd command and re-confirmed the performance issue. I did googling on it and I doubted it might be hardware issue or cable issue. Before I did any change, I boot my iMac with Ubuntu 16.04. I ran gnome-disks to check its performance and, surprising, it can reach 160-180MB/s. I couldn't believe it. I checked it again with "dd" command and copy large file as I did on Mac OS. The performance on Linux is constant and it is about 180MB/s. I rebooted it to Mac OS (no hardware change) and re-tested it. The performance of HGST hard disk was fall back to 20MB/s. It looks that it is a software (or firmware) issue and I wonder if anyone may have any suggestion on it.