Run a program which writes to disk and the changes won't necessarily be saved immediately. Windows holds them for a short time in its write cache, then applies multiple writes in succession to improve performance.
While that's a smart speedup move, it can also cause problems, for example if you unplug a removable device before all the changes have been saved.
FFB is a tiny portable tool which tells Windows to flush the write cache of one or more storage volumes immediately.
The program accepts multiple drive letters, drives or mountpoints, then uses a standard Windows call to write any cached changes to disk. Examples:
FFB C D E
FFB C: D: E:
FFB C:\ D:\ E:\
There are also switches to dismount the drive as well (an "eject" action, effectively), optionally forcing a dismount by closing any open handles.