Wrong disk space usage give me ftp file creation problem

[RodEz]

Hi,
i've a weird problem on a ubuntu server installation. First of all, df -h give me

Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb1              18G  1.8G   16G  11% /
none                  1.5G  192K  1.5G   1% /dev
none                  1.5G     0  1.5G   0% /dev/shm
none                  1.5G   80K  1.5G   1% /var/run
none                  1.5G     0  1.5G   0% /var/lock
none                  1.5G     0  1.5G   0% /lib/init/rw
/dev/sdc1             917G  878G     0 100% /mnt/1tb

as you can see, the last disk show strange value, 100% usage but size and used seems to say otherwise.
Until now, this doesn't give me any problems, but now i realized that i can't create new file or folder in such mounting point.
But that's not all, root can still create file and folder, only the user can't make new files or folders.
This last fact has left me a little in doubt, or the disk is full or it isn't, it can't be full for the user and free for root. I can't understand what is happening. Somebody have any suggestion ?
thanks in advance

[hazel]

Some of your disk space (5% by default) is reserved for root use. According to the mke2fs man page, this allows root-owned daemons such as syslogd to continue to function correctly after non-privileged processes are prevented from writing to the filesystem.

[RodEz]

thanks. So, looks that it is an archive disk i set reserved space to 1%.
thanks a lots