Apple - VirtualBox VERR_ACCESS_DENIED

I think you may be hitting El Capitan's System Integrity Protection [SIP] which is preventing access..

Borrowed from How do I disable System Integrity Protection (SIP) AKA "rootless" on OS X 10.11, El Capitan? ...

Apple's documentation covers disabling SIP, About System Integrity Protection on your Mac.

An article on lifehacker.com lists these steps:

1. Reboot your Mac into Recovery Mode by restarting your computer and holding down Command+R until the Apple logo appears on your screen.
2. Click Utilities > Terminal.
3. In the Terminal window, type in csrutil disable and press Enter.
4. Restart your Mac.

in case this helps anyone, I have been booting my bootcamp partition via vbox since probably OSX Lion. when SIP came along, I had to adjust my vbox run script to do the chmod's as described elsewhere, but today I had to re-create my pointer VMDK (VBoxManage internalcommands createrawvmdk...) because my drive numbering changed (/dev/diskN) and I could not without temporarily disabling SIP... no chmod'ing would make a difference. After creating the new VMDK, I reenabled SIP, and am back to chmod'ing before virtual startup/after each host reboot. HIH

Try:

sudo chown $(USER) /dev/disk0s4
sudo chmod 660  /dev/disk0s4
chmod 600 ~/RawWinDisk.vmdk

after each MacOS restart I need again chown /dev/disk0s4 to start Windows VM normally.