I've searched and read many instances on this board of problems with the MicroSDHC card that ships with the Droid X. I had a similar problem. My card came pre-installed (16GB) and the phone arrived on 10/2/2010. The phone and card worked flawlessly until today 10/5/2010. I got an error saying the card was "blank" this morning when I turned the phone on. I've lost all my music and pics and a few apps. I tried some of the remedies on other postings on this board to no avail. I took a 2GB Micro SD Card from another phone, inserted it and powered up the phone and it works just fine. The 2GB was a Sandisk card which I have used over two years in another phone.
I am starting to get the impression that these "no name brand" cards, probably bought in bulk, are the culprit. I will be going by the local Verizon store (30 miles away as I live in a very rural area) and attempt to get them to exchange the card that doesn't work.