next time this happens, I would boot to the recovery console and run a chkdsk on the drive. If chkdsk runs at boot, it means there is a flagged dirty bit and something might be wrong with the drive, bad sector, etc., you need to run this to clear the dirty bit, but sometimes it must be done from the recovery console (it is more thorough)
NTFS will check for this Hipster, but can perform on the fly repairs, so you dont always see it, unless the problem couldn't be fixed. FAT32 on the other hand will be flagged dirty and have to have chkdsk run to repair.