The chassis SC846 can use 1x3.5 HDD (fixed) with an optional tray, but at what location are these trays used?
Furthermore, we have looked at your optional part list:
http://www.supermicro.com/products/chassis/4U/846/SC846A-R1200.cfm which indicates that for a single 3.5" fixed HDD tray, RA cables are required.
What are "RA cables", and how do you them? Are there any options to mount the Slim DVD-ROM on the front side of the chassis?
The Slim ODD (optical disk drive) is mounted in the back. RA cable stands for “Right Angle cable” because of its angle. You need to use this cable if you install such a fixed drive:
Optional right angle SATA power extensions:
CBL-0143L: 1x 4pin to 1x SATA S-RA SATA power extension
CBL-0082L: 1x 4pin to 2x SATA S-RA SATA power extension
Optional right angle SATA cable:
CBL-0231L 70CM ROUND S-RA SATA cable