VirtualBox - HDD Partition Passthrough?

Just wondering if there’s a way to passthrough a HDD partition to a VirtualBox VM.

There is! It’s a just a single command too:
VBoxManage internalcommands createrawvmdk -filename "</path/to/file>.vmdk" -rawdisk /dev/sdXY
change /dev/sdXY to the partition. Ex, /dev/sdc4. You have to do it as root though.
Then mount it as a virtual hard drive