If you are seriously trying to process 75Gb of production data then I would not recommend the Sandbox which is just for playing around.
However to answer your question you should be able to use VirtualBox to create new hard disks and allocate them to your Linux virtual machine. This would require some linux knowledge – but I am sure you can google for “how to add a hard disk to a linux box”. Then you can tell the Sandbox – to use the new disk space for Hadoop”.
This page may give more details. https://www.virtualbox.org/manual/ch05.html#vdidetails