Almost 80% of all of these defective SD card threads have common solutions. I will list these below.
1) A corrupt SD card:
SD cards, especially the Micro SD generation can be damaged very easily if you are not careful. Any physical damage to the card will or the gold contacts will cause it to become unreadable. You know your card is corrupt when the computer cannot explore the card. SD cards can be damaged by forcing them out of the device they are in. If your SD card has become stuck in your computer, camera, MP3 player or printer.
It is best to take the device to a professional to have it removed, rather than trying to force the card from the device using probes, such as paperclips or screwdrivers. If the casing for your SD card becomes damaged or you accidentally damage the inside of your card, you could lose the data currently stored on the card and render the card useless in the future. Also, once it is corrupt, you cannot even sell it for parts or use it as a paperweight because of its low weight.
If the gold contacts are dirty in anyway, this will mean the device will not be able to read it. Using a finger can scratch the contacts and make it corrupt. Instead use a cloth, microfiber if you have one. The reader, (device) will also accumulate dust over time. Clean out the reader with an earbud. This may also solve this issue.
ADDITIONAL: Thanks to hb-kevin for mentioning that you can " clean the contacts with denatured alcohol. It's a cleaning alcohol that is perfectly safe on electronics such as computers, motherboards, and phone parts. If all else fails, try giving the contacts a gentle scrub with some alcohol and a soft toothbrush. "
Removing an SD card from a computer without properly ejecting it can cause software damage to the card. Removing your SD card while your computer is trying to read it or removing the card from a camera or MP3 player that is powered on could cause your SD card to lose the data that is stored on it and force you to reformat the card before using it again. Many tools can be found online to help you with this pursuit.
SD cards are enclosed in a small plastic casing that can be easily damaged if the card is left unprotected in a pocket, chewed on by the dog or stepped on. If the casing for your SD card has become damaged, do not put the card into a device (it might become stuck). Instead, take the card to a data recovery center in your area and have a professional recover any files you might need from the card.
On many SD cards (not many micro) there is a physical switch on the side of the card. This is a write lock. This means that if the switch is up and locked, the sd will refuse any data trying to be written to it. As a result, no data will be copied to the card - if it is in a computer, you will get a popup dialogue notifying you that it is write protected, just flick the switch.
The write protection could have also been enabled in a computer. To disable it, just right click on the SD in my computer and then click properties. Then go to attributes. If "read only" is checked, uncheck it. This could be the source of your problems.
Please read this before posting another SD thread.
If any further help is required, please leave it down below.
EDIT: Thanks to Cooleagle for making this a sticky. Because of the stick, many more people will see this first and not have to create a thread.