vdi to hdd conversion is essentially export between two types of VM disk drives, to be more specific between VirtualBox (.vdi) and Parallels Desktop (.hdd) files, usually in effort to migrate between virtual systems.
In case that you have access to Parallels Desktop, you can just import .vdd there, no conversion required, the software accept these formats just fine. But if you really need to convert vdi to hdd, just use the free VirtualBox from Oracle. You can do this through the Virtual Media Manager that can be found in the File menu.Open VDI file Open HDD file
Mac version of the popular virtualization software
A virtualizing solution for small businesses and professionals
Virtualizing software for Linux operating system
A multi-platform x86 virtualization software package
Free system virtualization platform for Linux operating system developed by Oracle
Free open-source x86 virtualization software package for Mac