Don't roll your own crypto. Just don't. You can never be sure it's secure, and the first you will hear of it being broken is when Anonymous posts your company secrets on Pastebin.
Use the tools that already exist (e.g. dm-crypt/LUKS for Linux systems, or BitLocker for Windows systems). They are well understood, and any reasonable security auditor will know what they are and that they work.
If the security auditor insists on this less-than-ideal setup, fire them for incompetenceincompetence.