hm..?

as I said: 'i just flashed a 1.8.1.7pre3 into it '

I mean that i flashed it, so just enter restore mode of the wl-hdd:
power off, hold the button, power on till power led begins to flash then release the button and start firmware restoration at your pc

after it's finished turn off wl-hdd turn it on, start system
after that make sure you load the firmware defaults

at the moment you will NOT be able to access your harddrive

if things go well perhaps WL-HDD will be incorperated in current WL500g firmware

the lan port port is the 'WAN' port with the WL500g firmware (thus eth1)