It's really a scary interesting read. https://www.dni.gov/files/documents/ICA_2017_01.pdf
Contrary to claims made by various liberals, I know of no security audit process, postmortem penetration assessment, or security firm that would ever be satisfied that an incident had been analyzed with "High Confidence" (or any confidence, really) if they could not completely examine every component of a compromised server down to the metal. Not just hard drives, but every module of firmware, eprom, or any other component or attached peripheral with persistent storage, not limited to, but including printers and routers.
Especially if, as has been claimed, a sophisticated state-actor was involved.
For all we know, the DNC servers remained compromised well after the assessment and may still be compromised.