Using other block device when conecting Android device to computer as USB storage
On my device (Sony Ericsson Xperia X10) it is, for example:

echo /sdcard/debian.ext2 &gt; /sys/devices/platform/msm_hsusb/gadget/lun0/file


After it the content of file becomes available on the host. Can write to sdcard file without unmounting it from android.