Virtual Machine (VirtualBox) -- How to access files from the hosting machine?

There are several ways to accomplish this.

You can use VBox's own Shared folders feature.

For this: Go to Settings of the Virtual Machine and then select: Shared Folders.

Click on Add Shared Folder button. Specify folder path and folder name.

In Virtual Machine you go to \\vboxsrv\

you would see your folders.

You can also Create a network share and access that share from within B to A.

I assume you have your VM up and running. Follow these steps:

1. On the VM window, Go to Tools -> Insert Guest additions CD Image.

2. You'll see a new CD drive in My computer. open it and start "VBoxWindowsAdditions"

3. Follow setup to install guest addition on the Guest machine. Reboot the guest.
4. Next, open VM settings:
5. Go to Shared Folders -> right click -> add shared folder -> add the folder you want to share:
6. Your shared folder will appear under My Computer in the guest machine.

You can follow similar method for other operating systems too.

Another option is just to copy files:

Select Virtual Machine and go to Settings - General - Advanced, there you can configure Shared clipboard and Drag'n'Drop support. I selected Bidirectional option for both.

Now you can drag-drop files between Host and Guest, also you can copy text in Host machine and Paste it on Guest VM (and vice-versa)