OK, I've solved the problem (this is what you get for custom-compiled kernels

). It's exactly the same situation as shown here:
https://www.mobileread.com/forums/showthread.php?t=60329
In short: you also need to have:
Code:
CONFIG_SCSI_MULTI_LUN=y
enabled under Linux.
Thank you for all your help and Calibre kovidgoyal!