Because HDtach bypasses the OS and the filesystem so its only useful to test the drives/cableing as being sound.

ATTO tries to bypass the disk cache using an OS level filesystem commandl they call the write_thru_flag which Microsoft for years did not support thru a bug. They found it and fixed it first on XP and has now implemented it on win2k also in sp3. Luckily nothing I have ever seen in real life uses it anyway excecpt ATTO and file copies using windows explorer. It also does not effect the writes at all just the reads.

But because hdtach bypasses the OS and the filesystem then cluster size and a host of other variables that effect real life performance are also bypassed. HDtach is more of a theoretical limit of what your top performance could be and not what you will probably get.

Tex