I don't think the USB drive is write protected, because you can see the logs. "dir /s \" can be interesting too. Maybe update to the the latest firmware 2.35.
One problem in your script: "md \zzz" should be "md \usb\zzz", but I guess you have already created the directory. Also check that it's empty. Sometimes files are write protected in Windows, which you can change with "attrib -r filename" and which you can see with "attrib" (on a Windows machine). Maybe wait a bit longer if the USB drive needs more time to write the files. Windows should mount USB drives without caching, so usually no need to unmount it.
Writing to the \secure directory and subdirectories is possible too. And with the USB startup trick there is no danger to brick the scope, if you have made backups of the files first, because the scope resets, if the application crashes on start, and then it would go into an infinite reset loop. But with the startup file you can restore a non-working executable again.
You could test writing with something non-critical, like replacing the startup image:
copy \secure\infiniivision\splashImage.bin \secure\infiniivision\splashImage.bin.org
\windows\compileImageForSplashScreen \usb\yourimage.png \secure\infiniivision\splashImage.bin
yourimage.png works with 800x480 pixels, 24 bit color, no transparency.