How should I transfer files from Windows to Ubuntu installed on Virtualbox? When I plugged in a USB, it only pops up in Windows.
How can I see it in Ubuntu?
There are 2 ways, which I normally use
Before booting up Ubuntu, inside Virtualbox Ubuntu VM settings, specify a share folder. Then after logged in to Ubuntu, create a new directory for example
Before booting up Ubuntu, add a new Network adapter and select 'Bridged Adapter'. Then after logged in to Ubuntu, run the command
Which is the host, and which is the guest? Either way, Virtualbox has a sharing mechanism you can use; just install the Guest Additions as described in the documentation then setup a share in the VirtualBox configuration.
In any case, you can easily make the USB key visible in the guest. From your question I imagine you run Ubuntu in a guest and the host is Windows.
Whenever you insert an USB key, if you want to see it in the guest proceed as follows:
This operation is equivalent to physically inserting your USB key in a USB port of your guest machine.
Otherwise, the other options are