Actually; the tool I sent you was slow SLOW to run.... Try this Windows command instead (you need to run this from a command prompt with Admin level).
Code:
fsutil file createnew kobo_drive:\bigfile 1000000000
This will make a 1GB file called bigfile in the root directory of the Kobo.
Repeat as needed (with appropriate values) to fill up the Kobo (or as close to fill up as you want).