If you are using a DSLR camera, like for example a Canon 60D, there may be an option to use a "Low-Level Formatting" on the card. This should make it writable when putting it into the computer.
To test this (and confirm it works) I did the following:
Canon 60D camera body
Sandisk 32GB SDHC Extreme Card
Macbook Pro 15" (mid 2010 model) with built-in SD card slot.
Place SDHC card into camera (SD card may work as well). Navigate through menu until you arrive at settings that allow the camera to format the card. There may be an option to format it "low-level", which you would then select.
-Note that all files on card will be erased permanently.-
After the camera formats, eject the card from the camera, and place inside the built-in SD slot on the Macbook.
The card should automatically mount, containing both read and write priviledges.
Hope that helps.
Owner/Founder HotTips! LLC
Some interesting answers on here but before i began to stab needles into my reader slot i thought i better try something less physical. My card will only read on my macbookpro 10.10.3, this happens intermittantly. putting the lock switch half way made it read and write (unless it was pure chance) but i still couldnt format the card. I eventually put it in an iMac 10.6.8 and erased it. it didnt work in my macbookpro until i worked my way through the options in disk utility (on iMac) to the 7 pass erase. now my card works as normal in the macbookpro reader slot. I dont know if this issue has anything to do with the fact that it was formatted in a camera (Sony NX5), but its very frustrating as i do not want to rely on other peoples computers for basic tasks such as this!