These words are used, so no worry about using them.  

flash removable disk
flash memory stick
flash drive

pen drive
USB Hard drive (well, these are bigger with capacity of even 300 GB (or more), with the size of a cigarret packet.

Sometimes referred to as a jumpdrive, the pen drive is a portable USB flash memory device that can be used to quickly transfer audio, video, and data files from the hard drive of one computer to another. With a construction that is small enough to fit into a pocket, the pen drive derives its name from the fact that many of these USB drive devices resemble a small pen or pencil in size and shape.

http://www.wisegeek.com/what-is-a-pen-drive.htm

http://www.blueunplugged.com/c.aspx?c=40661

 As you can see, they use the term galore, in the wired magazine (so people into computer/technology use it.)--->

    pen drive site:www.wired.com

 Don´t know, tough; if it´s widely used in everyday conversations.     I think so, but let´s wait some comment on that. 

PP.