Good morning ma5646,
Congrats on getting a Samsung Stratosphere! I LOVE the keyboard on this device. May I ask are you having issues with another memory card other than the 4GB pre-installed card that comes with the device? If so then the steps that Ann154 suggested would be the best route to go. If you are able to extract your files from the memory card onto a computer then the card would need to be reformatted. Once you have formatted the card, place the card back into the device to confirm that the error message is no longer appearing.
If the issue is with the pre-installed 4GB memory card that comes with the Stratosphere then I encourage to try the previous steps posted on this one as well. If you are still getting the error message on the new card then we can look into warranty options since you recently purchased the phone. Once you have provided some additional insight on the issue then I can further assist in a resolution for you.
I am having a similar problem. It is the pre-installed 4GB memory card that came with the Stratosphere. I was able to get the files off the card and onto my computer. I then reformatted the SD card. It worked for a couple of hours and then I got the dreaded "SD card not detected" message. Is this a damaged card? bad phone? o/s bug? Help...