Android 4 recovery: /udisk/ vs. /sdcard/ directory

I've got an Android 4.1-based device (PlayJam GameStick) whose official firmware updates had two files: 1. Update.img 2. factory_update_param.aml The factory_update_param.aml contains instructions to automatically install the img file:

--update_package=/udisk/GameStick-Software-v2071.img

Copying those two files to SD card and booting into recovery leads to a "aborting finding installation package" (from my mind, probably worded a bit different). In recovery, the .img file was available in the /sdcard/ folder and not /udisk/. What is the /udisk/ folder? How can I get files to appear in there?