My device bricked (for now ...).
It worked fine with 1.10b5, didn't accept the 1.10b6 first, but I suspected an error while download or unzipping so I dl'ed it a second time with different software.
This time it accepted the fw file itself, but after reboot the power and network LEDs keep flashing, HDs do not spin-up. No access possible, my router recognized the DHCP request (the device showed up as active network client) but there's no access to web GUI now.
First, I'll take out the drives and backup their contents. Second ... if there's no automated recovery tool which force-push firmwares through some sort of bootloader, I have to make a serial console cable and recovery process, as described
here. If someone knows a better way, I appreciate it

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*edit 2011-04-04:
Today I gave it another try to boot-up, this time without HDs. Power and network LED keep flashing. Unfortunately, it doesn't show up as "active" anymore in my router, neither it gets its IP adress from the router.
So it looks I have a "recovery case" here.
edit*
//HSishi