Debian :: Root Partition Full But It Is Not?

Got problems with apt. So I discovered if I remove some stuff from root partition apt works normally again.

So I tested to copy 1GB file to root, but in the middle says disk is full, but there should be 1.8GB free if I type df -h.

Code:
Select alldf -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs          4.0G  2.2G  1.8G  56% /
tmpfs            10M  4.0K   10M   1% /dev
/dev/md0        4.0G  2.2G  1.8G  56% /
tmpfs           492M     0  492M   0% /dev/shm

[Code] ...