Message of insufficient space on hdd.

Router Asuswl500gp2, Oleg's firmware 1.9.2.7 - 10, hdd Seagate 2Tb with external power. The router settings are according to this tutorial: http://wl500g.info/showthread.php?t=10307

I created a folder mkdir /tmp/harddisk/share, copied there about 100Gb and got a message for insufficient space on harddrive

[admin@Wl500gp2 root]$ du s /tmp/harddisk/share
104844952 /tmp/harddisk/share

also mounted folder big, the problem is still there

[admin@Wl500gp2 root]$ mkdir /tmp/mnt/disc0_3/big
[admin@Wl500gp2 root]$ chmod ugoa+w /tmp/mnt/disc0_3/big

there is plenty of free space on the drive:

[admin@Wl500gp2 root]$ df
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/root 3072 3072 0 100% /
/dev/discs/disc0/part2 985720 65988 869660 7% /opt
/dev/discs/disc0/part3 1921383312 104976180 1816407132 5% /tmp/mnt/disc0_3

Filesystem is ext3

[admin@Wl500gp2 root]$ mount
/dev/root on / type squashfs (ro)
none on /dev type devfs (rw)
proc on /proc type proc (rw)
ramfs on /tmp type ramfs (rw)
usbfs on /proc/bus/usb type usbfs (rw)
/dev/discs/disc0/part2 on /opt type ext3 (rw,noatime)
/dev/discs/disc0/part3 on /tmp/mnt/disc0_3 type ext3 (rw,noatime)

How can I check if there are restrictions for folder size and what can be reasons for that?
Why do i get a free space message and how can I fix it?