MacBook Pro mid 2012 13.3 HDD cable performance I am helping a friend to fix her MacBook Pro 13.3 performance problems.

It was barely booting, and terribly slow for disk seeks. Then I replaced the HDD.

It was perfect with new SSD and El Capitan installation but in the third boot it become same bad as before.

It is so weird but I realized when I disconnect the HDD cable for a while then reconnect it, it works good for 3-4 boots then it become extremely slow again. I know that this model has a cable problem but I can't understand what kind of problem is this. I thought it could be a firmware problem and I don't want to waste money on buying a new expensive cable.

Any advises on that?
There have been HDD cable issues with every model of non-Retina MacBook Pro (13-inch). This includes the Mid-2009 model, all the way to the Mid-2012 (today's model).

The fix is to simply replace the cable. You can find them all over the place, and it just requires a small cross-head screwdriver.

The issue is thought to be related to the cable weakening and losing contact with the socket on the board. In fact, Apple recommends installing small rubber pads on the hard drive cable after replacement.

If an Apple technician replaces the cable for you, that should be included. If you order the part elsewhere, the pads may come with it. You can see that demonstrated in this picture.

I recommend following iFixit's guide and ordering the part there. It will save you a lot of headaches with intermittent hard drive issues if you leave the cable as-is.