SMART Warning + disk full

[mstankie]

Hi there!
I've got a QNAP NAS TS-239 Pro II+ with 931,5GB RAID mirrored disk. Last week I got a SMART Warning (Reallocated_Sector_Ct -raw value 3, threshold 036), but the HDD I/O Status is Good, and "there are no errors found". At the same time the disk got full (0 KB), however in the disk usage panel, if I sum all used disk space it gets 740GB, so I don't know where are my almost 200GB?
If I delete some files, in a few minutes the free space is lost -no of my workmates who use the disk reports any data upload.
Some idea what's up? -Thanks in advance.