Why doesn't Android recognize my SD card when my computer does? There appears to be a problem with my micro SD card. It is 8 GB (really 7.4) is not recognized/mountable by my phone. When I put it into an SD card adapter, I can see everything on the card perfectly. Note this this is the original card provided by Samsung when I bought the phone.

However, I have a 2 GB SD card from PNY and it works without issue.

What does this mean? Why can one card be read while the other (which has been working for about a year now) cannot?

The old post can be found in the edit revisions.
There are multiple approaches to this. Not all of them are available on all devices, so you might have to "skip" some. I will list them in the "order of their convenience", i.e. easy and non-destructive things first:


Check Settings→Storage for some entry to delete/format the card
Connect your device to your computer via USB. If you're lucky, the card will be offered to it in UMS mode. Then use your computer's tool to format the card (take care to not use NTFS as file system, Android can't handle it. To be on the safe side, cards usually ship with FAT file systems)
Remove the card from your device, and use a card-reader to attach it to your computer. Format it using your computer's tool, as described in the previous example.
As many manufacturers decided to replace UMS with MTP, if none of the above works for you a downgrade to an Android version before 3.x might bring back UMS support. But that should be left as the very last ressort; after all, a card reader comes cheap. Depending on the device, there might even be possibilities to enable UMS using a hack or a specific app (provided the device is rooted), see e.g. Enable USB Mass Storage mode in LG Optimus 4X HD.