unix - free disk space disappeared on SSD

Today my disk got full. It was surprising, because I've been running the same automated script for a few months, with plenty of space, and no issues.

I don't understand where it's gone. du reports only 26 GiB used, which is what I expect.

root@data3:/home/upload# du -hs /
26G /

But df reports me that I'm using 46 GiB:

root@data3:/home/upload# df -h
Filesystem      Size  Used Avail Use% Mounted on

rootfs           52G   46G  4.0G  92% /
/dev/root        52G   46G  4.0G  92% /
devtmpfs        2.0G  4.0K  2.0G   1% /dev
none            395M  224K  394M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            2.0G     0  2.0G   0% /run/shm

I looked everywhere for big files, and didn't find any other than expected.

Here's is fsck results

root@debian:~# fsck -fv /dev/sda1
fsck from util-linux 2.20.1
e2fsck 1.42.2 (9-Apr-2012)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information


   46418 inodes used (1.36%)
     188 non-contiguous files (0.4%)
      77 non-contiguous directories (0.2%)
         # of inodes with ind/dind/tind blocks: 0/0/0
         Extent depth histogram: 40238/63
 5782304 blocks used (42.45%)
       0 bad blocks
       1 large file


   33182 regular files
    6372 directories
       0 character device files
       0 block device files
       1 fifo
      22 links
    6853 symbolic links (6107 fast symbolic links)
       1 socket
--------
   46431 files

This is a SSD drive. I've been writing/erasing about 15 GiB a day since July.

Where did my 20 GiB go? What should I check?