"USB stick is already connected when my system boots" - there is the problem. OS/2 systems do not expect usb drives to be attached at boot.
That may depend on which USB drivers you use. ArcaOS has no problem with USB sticks that are attached at boot time (USB stack 11.17, or later, and I think that Lars' drivers do the same). They are simply not enabled until the system finishes booting. Then the built in AN Removable Media Monitor (REMM) widget mounts them. REMM has actually replaced the older USBMON program (that needs to be started at boot time to auto mount USB sticks). There is a duplicate file name, in a different directory, that is used for printers. (sorry, I don't remember all of the details from the ancient past). I think that USBMON also had a different (but similar) name in earlier versions of OS/2 (duplicated for printers). The really old way, was to use the LVM command, discussed earlier, but that still works. Pre-LVM did all of that in a different way.