After installing Mavericks, MacBookPro9,1 (mid 2012 MacBook Pro 15inch) and Western Digital MyBook Studio II 6GB (Raid1 - 3GB+3GB) connected via Firewire with 2 partitions reappeared as single empty MyBook and exposed EFI partition.
Hi JazzFruit,
Sorry to hear you cannot access your files. I recommend you to contact tech support directly on this case.