Hi.
I found a funny I/O slowdown with devb-umass and io-usb on QNX 6.3.0.

The target file system type is type 11 (DOS 32-bit FAT).
Because It's '/dev/hd2t11' in my case.

I measured I/O speed few times.
Then I got the problem alternately.

1st time: No problem.
2nd time: Got the problem.
3rd time: No problem.
4th time: Got the problem.

Attached umass_speed_wofw.sh is a test script for this problem.
I used 'dd' and 'time' command for the measurements.
I/O size are 256x65536[bytes], 512xx65536[bytes], 1024x65536[bytes], 2048x65536[bytes], 4096x65536[bytes].

The result show in rwresult_wofw.png.