Ubuntu – Can’t access VirtualBox shared folder

You have to mount your folder on your VM.

First you need to install Guest Additions (although I already did this during the installation).

1. Start your VM
2. Devices > Insert Guest Additions CD image...
3. I had to manually mount the CD: sudo mount /dev/cdrom /media/cdrom
4. Install the necessary packages: sudo apt-get install make gcc linux-headers-$(uname -r)
5. Install the Guest Additions: sudo /media/cdrom/VBoxLinuxAdditions.run

Now you can mount your share using:

mkdir ~/new
sudo mount -t vboxsf New ~/new

Where New is the name of your shared folder.

Now you can access the shared folder at ~/new.

Note: this is not permanent. To permanently mount your folder you should add the following line to /etc/fstab (sudo nano /etc/fstab):

New /home/user/new vboxsf defaults 0 0

Obviously you should replace user in /home/user/new by your own username.

You have to change the "Guest properties" to modify the mount prefix and base directory used for the shared folders.

See https://www.virtualbox.org/manual/ch04.html#sf_mount_auto
and https://www.virtualbox.org/manual/ch04.html#guestadd-guestprops

An important note, though:
Both properties detailed in the first link have default values when not set or cleared:
/VirtualBox/GuestAdd/SharedFolders/MountPrefix defaults to sf_ if not set.
Set /VirtualBox/GuestAdd/SharedFolders/MountDir defaults to /media if not set

As a consequence, you have to set MountPrefix to / if you do not want the sf_ prefix, as trying to set it to "" will in fact clear the property and put it back to its default value.