There's a section on this in the latest 6.4.0 release notes (see the
Download centre on the website):

Required device properties for the Power-Safe filesystem

The Power-Safe filesystem was designed for and is intended for
traditional rotating hard disk drive media. It operates by moving the
on-disk filesystem state from one stable view to another stable view
using copy-on-write (COW) to relocate modified blocks. To finalize this
transition, all dirty blocks involved in the new view must be committed
to persistent storage, and then a new filesystem superblock/root
referencing the relocated blocks is committed.

This provides power-safe robustness, because at any point in time either
the old version is completely accessible or the new version is
completely accessible (with no live data being overwritten in between).
Thus to mount as read-write on a given device, that device must have the
following properties:

    * one of the following:
          o The device may buffer write data for performance reasons,
and the return from a WRITE may not necessarily indicate the data is
committed to permanent storage. But such a device must implement a
FLUSH/SYNC command that forces any cached or buffered write data to
persistent storage, and doesn't return until it's guaranteed that all
data is stable across a power-loss.

            or:
          o The device doesn't buffer write data, and operates in a
strict write-through manner, where return from a WRITE is a guarantee
that the data was immediately committed to persistent storage. Such a
device doesn't require an additional FLUSH/SYNC command.
    * and both of the following:
          o The action of writing to one data region (an advertised
device sector) can in no way damage the contents of any other region,
even under conditions such as power-loss, vibration, temperature, etc.

            and:
          o Data that has previously been reported as committed to
persistent storage remains stable until explicitly overwritten. The
device may implement facilities such as bad-block remapping or
wear-leveling to support this requirement, provided that such activity
never causes loss of persistent data, even under conditions such as
power-loss, etc.


Steve Reid (stever@qnx.com)
Technical Editor
QNX Software Systems 

Thank you, I got it working. I didn't wnat a power-safe filesystem on the USB stick, I wanted one that can handle files 
larger than 2GB. This is the true value of the fs-qnx6 for us.

-Albrecht