Great idea. This is another common issue and I'd forgotten all about it.

To amplify - /boot and /home are directories used for mount points. When /boot and /home filesystems are mounted on them then the data written into them go into those other filesystems. If however the filesystem are not mounted for some reason you can still write in the directories - such writes take up space in the root filesystem instead of /boot or /home. You will need to go to single user (or possibly even have to use a rescue disk) to not mount /boot. However it is more likely your issue is in /home and you don't even have to go to single user. Just make sure no processes are using /home and you can umount it then check the directory /home with ls to see if it has anything in it despite being unmounted and can use du -sk /home after the umount to see how much it has in it.

@Wim Sturkenboom: That's exactly what it was!!! 45Gigs of data under /home...

Thanks for the help!!! I'll try and remember that one

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Pleasure. Just mark your thread as solved using the thread tools just above the first post on a (this) page

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By the way - you can try to avoid this problem from happening again someday by making sure the unmounted /home directory (the mountpoint) is owned and writable only by root. With /home mounted you leave the ownership/permissions as they are currently. By doing this if it fails to mount for any reason in future then you'll see attempts to access /home fail rather than writing into your root filesystem and will know to investigate the mounta failure at that point.