thanks, it helped ...
I knew about UDEV, but I didn't have any special rules about /dev/sd* devices, so I ignored it. But I got it now ...
If someone needs it:
I want my hotplug scripts to be able to recognize which device in /dev/ they got. /etc/hotplug/usb.agent delivers global variable $DEVPATH to every script.
so if I need to find out device name in a script, I can use $DEVPATH to find it. I use this only for "usb-storage" (block) device, but it can probably be used for other devices also.
when hotplug runs my_usb_script in /etc/hotplug/usb/, I should be able to find the device name with $DEVPATH.
USB_DEVICE=`udevinfo -as | grep -B 4 "$DEVPATH" | grep /sys/block | sed -re "s/^DEVPATH\s+'\/sys\/block\/(.*)'/\/dev\/\1/"`
USB_DEVICE is now /dev/sda or /dev/sdb or /dev/sdc... I had to put "sleep 5" before this, because hotplug/udev needs a litle bit to make a device etc.
I have USB-stick with a openssl key for a encrypted partition (dm-crypt). now when I put USB-stick in an USB slot, zenity asks me for a password in a popup windows and my encrypted partiton gets mounted. when I pull it out, the partition is again encrypted and umounted.