Hello,
I'm a rather frequent and loyal Sun customer, I think over time I must have bought just about every Fire model, and they all came with empty drive bays where no hard disks have been ordered. I never saw any «dummy» or unusable bay between the years 2000 and 2010.
Yes, empty drive bays where you can't insert a standard bare disk. You need Sun disk mount kits, which are not sold separately.
You both sure have made very contradicting experiences with Sun HW!
My experience with Sun is what Tonnerre described for 3.5" bays, and what Stanislav said for 2.5" bays...
You can find 2.5" trays on e-bay though (look for the corresponding Sun part number).
Best regards,
Alexandre