For the past week or two I've been having an issue where apps will say that my Hard Drive is full (not the startup drive, the one where my home directory is). iMessage is saying "Disk Full. Messages may be lost" and Spotify is saying "Unable to save to cache. Maybe the Hard Drive is full". The Mac sometimes stalls for extended periods of time (10 seconds+) while opening a file etc.
Looking in Disk Utility, all of my hard drives have adequate space (75GB on the HD in question). But I've just launched it now and it switches between 75GB and 148KB (so there's obviously an issue somewhere). I've ran Disk Repair on it but that found no issues. What else can I try?