For some time, every time my phone starts up it offers to scan and fix SD card errors but warns it may delete files. Everything is working normally but the message appears on every startup. Then I finally made my backups, ran the scan but nothing changed - still gives the message every time phone starts.
Some people suggest the problem may occur if you're using the SD card and then the phone crashes or runs out of battery. It's best not to have frequently used apps on the SD card. Also I fixed my problem by plugging the sd card into sd card reader on my laptop and running error checking there: http://www.ehow.com/how_6857598_check-sd-card-errors.html (strangely enough, it said no errors were found but suddenly the phone was happy).