Boot from an existing (real) hard drive
See manual, section "Using a raw host hard disk from a guest"
Basically, you'll create a special .vmdk file, using the command-line tools. On Windows, you do the same thing, but you use UNC paths rather than the /dev/ style descriptor. (I went right click > Properties > Tools(tab) > choose the drive > Properties > Volumes > Populate > see "Disk" field) You can use the listpartitions command to make sure it's the right one.
Setup folder sharing (assuming Linux guestOS)
1) install Guest Additions. 2) mount -t vboxsf <vbox-foldername> <mountpoint>
Boot into a bootable USB drive
[anywhere] this will never be officially/easily supported... though there are workarounds:
[in Windows] you may be able to have it show up as a floppy drive.
[in Windows] VBoxManage internalcommands createrawvmdk
[in Linux] VBoxManage internalcommands createrawvmdk
remember to unmount before booting, however
[anywhere] if you disable USB 2.0 [1] [2], then Plop Bootmanager will actually work (albeit slowly)
hrm... this might actually be a bug with VBox itself?
various operations regarding .vdi files