Hello Isabelle.
I saw similar problem couple days ago - I thought my external USB drive collapsed...
I developed my pictures krom K-7 in Lightroom 2.4 and some of them exported to .jpg. Next day I was not able to read (or to show) some of them (.
pef and .jpg) neither in Lightroom nor in viewev (for example in Irfanview)... I had copy of files on another HDD - the files were of the same size, of the same date, but comparison showed some differencies... I formatted my USB HDD and made new copy of all files. I am now quite nervous reading your problem... Maybe bug in Lightroom 2.4???
Have a nice day,
Drahos
http://www.drahosfoto.sk