My username is "user", so looks like it is part of the 'disk' group..
user:$ cat /etc/group | grep disk
disk:x:6:adm,user
Below, I have showed a full listing of what happens. By doing everything as root, I managed to make some sort of progress (i.e. I ended up with a different error

).
!
! as root, unmount the MINT partition
!
root:# umount /mnt/tmp
!
! as user, try to create the vmdk file
!
user:$ VBoxManage internalcommands createrawvmdk -filename /home/user/myvbox/partition2.vmdk -rawdisk /dev/sda2 -register
!
! but it fails
!
Oracle VM VirtualBox Command Line Management Interface Version 3.2.8
ERROR: VMDK: could not open raw disk file '/dev/sda2'
Error code VERR_ACCESS_DENIED at /home/vbox/tinderbox/3.2-lnx32-rel/src/VBox/Devices/Storage/VmdkHDDCore.cpp(3503) in function int vmdkCreateRawImage(VMDKIMAGE*, VBOXHDDRAW*, uint64_t)
Error while creating the raw disk VMDK: VERR_ACCESS_DENIED
The raw disk vmdk file was not created
!
! try again, as root this time
!
root:# VBoxManage internalcommands createrawvmdk -filename /home/user/myvbox/partition2.vmdk -rawdisk /dev/sda2 -register
!
! now it works
!
Oracle VM VirtualBox Command Line Management Interface Version 3.2.8
(C) 2005-2010 Oracle Corporation
All rights reserved.
RAW host disk access VMDK file /home/user/myvbox/partition2.vmdk created successfully.
vector://home/user/myvbox
!
! now change permissions on the file
!
root:# chmod a=rwx partition2.vmdk
root:# ls -lat
drwxr-xr-x 2 user user 4096 2010-10-11 19:18 ./
-rwxrwxrwx 1 root root 537 2010-10-11 19:18 partition2.vmdk*
drwxr-xr-x 38 user user 4096 2010-10-11 17:48 ../
!
! VBox now lets me define a new machine using this file as the h/d, but won't let me START it
!
Failed to open image '/home/user/myvbox/partition2.vmdk' for writing due to wrong permissions
(VERR_VD_IMAGE_READ_ONLY).
Failed to attach driver below us! Image is read-only. (VERR_VD_IMAGE_READ_ONLY).
AHCI: Failed to attach drive to Port0 (VERR_VD_IMAGE_READ_ONLY).
Unknown error creating VM (VERR_VD_IMAGE_READ_ONLY).
!
! so go to the console and start VirtualBox from the cmd line.
! Now I CAN create & start with no permission errors BUT I get
!
FATAL : No bootable medium found! System halted