When setting up VirtualBox there are two time sinks that one can be easily get trapped by.

The first is getting an Internet connection. In theory, the Internet link of the computer that you are setting the Virtual Machine onto will be automagically linked to. Open Firefox and off you go.

Yeah; until it doesn’t auto set up. Then, it is poking around until you find the right combination of not-so-obvious tricks. And here is a synopsis that I just had to find and go through, and which hopefully will save you some time.

1. Shut down the OS. Leave VirtualBox running.
2. Click onto the Ubuntu VM
3. Up above or on the right panel, choose Network
4. Click Adapter 1 and set for NAT
5. Click Adapter 2 and set for Bridged Adapter, then select the adapter that has the link: e.g., en0, Wi-Fi (Airport)
6. Click OK.

As long as that is open,

1. Click on Ports
2. Click on USB. Make certain that the USB 3.0 Controller is selected. 
3. Click OK.
4. When you have the Virtual Machine running, click the icon on the bottom right that looks like a USB plug. Select USB Settings 
5. On the right side click the icon with a Plus (+) symbol on the USB Plug.
6. Select a plugged in drive from the list. If the drive that you expect to be showing isn’t showing, it may have been captured by the computers native drive system. Every Operating System is going to have a little different method for disconnecting an attached drive. Technically, it is called an UnMount. It gets a 2nd dose of complication because you can’t just say ‘unmount’…you must say umt (or something like that) and the name of the drive…which you probably don’t know. 
7. If you are only going to do this once in a while, go to your Disk Utility, find the drive on the list and click unmount.

At some future date, this article may be appended to include some more clues about unmounting, but this articles purpose was to get the 1st two points onto this virtual tissue.