1TB in 50 hours works out to around 5 MB/s.
It could be that your drive is running in PIO mode.
In addition to DLG, you could run a HD Tune benchmark. The read performance graph should be a smooth, monotonically decreasing curve. The access time graph should also have minimal scatter in the data points.
If the drive is in PIO mode, then the "curve" will be a flat line.http://www.hdtune.com/