Firmware Recovery TS-259 Pro+ with DSL getting Kernel Panic

[romeo-kilo]

When I try the Firmware Recovery for my TS-259 Pro+ using DSL-4-4-10-inetd.iso on a USB Stick I get the selections for "USB Flash Disk" and the "UNetboot in" menu. After pressing Enter at "Default" I see the four penguins and the following message:
>> Kernel Panic: VFS: Unable to mount root fs on 03:02 <<

Waiting for 10 mins didn't help, pressing Enter didn't help, trying other Linux versions from a stick had similar problems with kernel panic. Is there anyone who can give me a hint?

Thanks a lot!

[romeo-kilo]

by the way - I've dismounted the physical Hard Disks according to the documentation...