LR 2.7, 'Writing XMP metadata has failed'

I have several thousand photos from a scanning electron microscope that I use Lightroom 2.7 for archiving. The microscope doesn't create XMP data automatically so when I add photos to the catalog the first thing I do is fill in "Date Created" in the metadata and then write it to the file. I've done this with hundreds of photos.

Today I had 18 new photos to add. I entered the date and then clicked on the 'resolve conflict' button next to the Metadata Status box to write the data. But this time I get an error box that says: 'Writing to XMP metadata did not complete successfully' and then I have two choices, a button that says ignore and a button that says Show in Library.

I imported more photos later than this with no problems. Anyone have any idea what can cause this to happen? They are jpgs and the photos are accessible to LR, no question marks next to their names.

Some kind of permissions issue maybe?

That was it Rene, I didn't even think to check that since they are just another set of photos in a daily stream. But somehow these 18 were Read Only. Why I have no clue but I changed that and the metadata was saved as usual. Thanks for the suggestion.