i just tested this - results:
xferring xbmc while playing dvd - completed successfully
xferring xbmc in dash while playing music - completed successfully
xferring 4.83gb of divx videos while in dash playing music - completed successfully
only time i got it to freeze/reboot during these xfers was going in and out of menus which taxes stack memory with all the 3d transitions and rendering.
remember this dash was not originally designed to support ftp - it was an aftermarket add-on
playing music or a dvd should be plenty to keep you entertained while xferring things to and from the box. besides - ftp as far as i'm concerned is a temporary lan solution until my 'something better' idea can be implimented