When using a 2009 iMac, I noticed that the original hard drive was running very slowly. When I used the Blackmagic Disk Speed Test from the Apple App Store, the read/write speed was around 35 MB/s. After replacing it with a new 1TB hard drive (non-SSD), I found that the speed didn’t improve and remained at 35 MB/s. Why is this happening, and how can I fix this?
The 2009 iMac is equipped with an older SATA II interface and a maximum theoretical speed of 3 Gb/s (about 300 MB/s). If you are using an older mechanical hard drive, you may not be fully exceeding the bandwidth limit of the interface. Therefore, the hard drive’s speed might already be close to the maximum performance limit of the interface.
If you want to significantly improve speed, consider replacing the hard drive with an SSD. SSDs have much faster read and write speeds compared to mechanical hard drives, and the performance improvement will be especially noticeable on older hardware. Even if your iMac is equipped with a SATA II interface, an SSD will still provide better performance.