My problem was solved, not my HDD problem, my workstation was installed 7 4TB, the problem is this app manage cache very bad, I extend the cache to 500-3000MB, after that, it can handle most jobs when I multi download and over 5GB size per torrent, I kept monitor the memory usage, it will use 2.5 - 3Gb ram to cache when my torrents are 40GB total size, if more 40GB, then the HDD would little busy again, until the torrent was download completed and remove the completed torrent, then the cache will be change smaller. I counted if you want to download 10GB data at the same time, you need set the cache size to 1GB, then HDD would not busy anymore...