I had a similar problem with my SD card. I upgraded last weekend as well and then my ohone did not recognized my SD card. I went to the Sprint Store last night. I showed them the isssues below and they said there is a problem with Gingerbread upgrade and SD cards and they need to revert my phone to the previosu version from Gingerbread back to Froyo- I made an appointment for Monday to do this since they are releasing the EVO 3D this weekend.
Summary of issues:
When I clicked on my camera, to view/see photos, it says "An error was encountered with your SD Card" and I am unable to see my photos. It also happens when I open an email attachment. There is also a message saying: "insert SD card".
In Settings: Storage & SD card: it says the "SDcard is empty", yet in "manage card" there were Apps on it. I tried to connect my phone to my computer to copy the SD card onto my computer but the computer did not recognize the card- and there was no card on the phone via connecting the computer.
I took the battery out, and restarted, & rebooted the phone- did not fix issue.
We transfered my files via a card reader to a "backup folder" on the computer. After we transfered files we "checked for errors on the card" via " Properties: check for errors" and there were" errors on the card" that we fixed via the computer.
We went to "Format card" on the phone but the phone did not recognize the card, so we formatted the card on the computer, put the card back to the phone and the phone then finally recognized the card. We loaded the card with files, and it worked. My phone seems to be faster now too. I hope this helps you as well. This process saved me from going into the Sprint store on Monday and having them mess with my phone.