I see a lot of posts about writing to the SD card, but I believe my problem is different. I am unable to create the SD card during the process of building an AVD.
I had the same problem. After troubleshooting, I came to know that, it was because of my Windows is 32bit and my mksdcard is compatible with 64 bit only. Then I just copied a mksdcard of 32 bit to tools folder, and deleted the older one. It solved my problem.