I had an older iMac, which didn't have an SD card reader, so I ended up purchasing the Moshi Cardette Ultra, which has several slots for reading different types of cards, and this one specifically says it reads SDHC cards. It has USB ports too, so you could try hooking up external harddrives to it. Heres a link to a more detailed website. Hope this helps!
I know about that kind of hardware, but that doesn´t really solve my problem because I already tried one of those and it has the same problem I explained above.
And I don´t really want to buy something. I want my mac problem to be really solved. I bought it in 2010, and it always had this problem.
Intermittent problems are very difficult to define. You have to try all of the USB ports switch cables and try different enclosures enclosures. Have you tries all of this?
If you have, then an appointment at an Apple store genius bar is probably the best option. Bring the HDD(s) and SDHC(s) with you. Due to the nature of you problem, they may not be able to find exactly what is wrong unless the symptom ocurrs.