Mounting my new external Western Digital 500GB harddisk drive via firewire on Gentoo Linux failed with: ieee1394: Error parsing configrom for node 0-00:1023. At first I thought about a disk hardware failure. Testing the disk via Firewire on my second system showed the same message.
I began to doubt about the Linux firewire implementation as most of the 20 search results made me believe. Last try was to use the USB connection from the external case - and it failed as well: scsi 8:0:0:0: scsi: Device offlined - not ready after error recovery. usb 1-3: USB disconnect, address 15. Using a special IDE/SATA-to-USB adapter, I could access the drive.
Conclusion: The external case's chip does not work with larger drives. After some searching (nobody wrote the name of the manufacturer or the real product name anywhere, just "MAPOWER COMBO1HG") I found out it is a "MAP-H31C1" (Combo interface). Nowhere on the product page, nor on the datasheet is noted that the "external aluminium enclosure" has a size limit. Seems I gotta spend more money on something new.