You're going to go through the trouble to get a warrant for a raid and then apply such gross incompetence to the seized evidence? When was the last time you pulled a hard drive out of a functioning machine, found it to be empty and didn't immediately think, "huh, that's odd" before throwing it in the trash.

I guess it depends on whether the hard drives were the target of the raid or just a collateral of "grab anything that may be useful" mindset. They could already have gathered enough evidence without needing to waste time/money on digital forensics. Hard to say without specifics.

The fact that they went out of their way to assure him that his own hard drives were empty reeks of manipulation.

Color me highly skeptical.