Getting 20MB/s disk writes on my newly installed SSD

I installed my SSD that I've been using with no hiccups or problems in my desktop PC for a year, in my MB Pro, since I was using it more than my desktop, but I am getting super slow speeds from Black Magic Speed Test and I have no idea why.

In system information it says that I have 6 Gigabit Linkspeed under SATA. My firmware is up to date and I'm running the latest version of El Capitan.

Any help appreciated

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You likely need to replace the HD drive cable. The older cable was rated for SATA II (3.0 Gb/s) not SATA III (6.0 Gb/s). The early versions of this system had a SATA II HD drive.

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I went to an Apple store today, and the employe told me to run an Hardware Test, which yielded no results.

How can I tell if I need to replace the HDD Cable?


What is happening here is the dialog between the SSD to the System as well as from the System to the SSD is failing when you have a heavy data flow. Each read & write has a CRC checksum if the checksum fails to Ack the given data block it's retransmitted. Think of it this way, think about having a conversation with a half deaf person so every other word needs to be re-spoken. This is what is happening here, AHT won't catch this as it doesn't put enough strain on the connection unlike Black Magic or Drive Genius stress tests.


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