...actually, the plastic drive caddy is not required, or needed to mount additional hard drives in the 8930.  The drive bays are secured with two fasteners each and when removed, a hard drive can be inserted and secured with additional fasteners on the sides of the drive bay (four total per bay).  3.5 inch drives fit easily, but smaller 2.5 inch drives, including a solid state drive, will require an adapter to fit properly.  Aapters can be found on Amazon, as can an assortment of fasterers for computers.

This forum is a trove of information regarding anything and everything involving the 8930. Especially worth investigating, IMHO, is info regarding installing a M.2 NVme SSD for a VERY fast C: drive.  I like the 1TB size.  Fan noise, heat, etc, if annoying, is easily cured and again that info is available herein.

Enjoy the 8930, it's a hotrod of a computer. As always, YMMV.

The XPS 8930 does not use or require hard drive rails. The hard drive mounts directly to the hard-drive cage using #6-32x1/4" screws (4). See the Service Manual at dell.com/support.

Thank you very much.  I'm happy to hear it's something simple.

I appreciate the thorough response.  Thank you.  I will be attempting to mount my hard drive per your instructions.  I will also reference the manual.

Regarding a SSD, I do have one (512GB) as my boot disk.  I have one additional, typical hard drive in there, but would like to add more storage.  Hence my question about the bay, as I thought I needed a hard drive rail.

In buying this computer, which functioned off an SSD, I had hoped for a silent unit.  Akin to my apple laptop, which is considered solid state.  However, unfortunately, this PC is VERY loud with fans working at seemingly full speed upon startup and onward (doesn't require the computer to be working hard for the fans to come blasting on). I realize none of this fan noise is coming from the hard drive, rather the 'main computer' fan, but nonetheless very disappointed in that aspect of this hotrod.

Thanks again. 

@MissleFaceOne more thing I should have mentioned. You should get a right-angled SATA cable for your hard drive. The side panel may not close completely if you use a cable with a straight connector. Look at the cable that you currently have connected to the hard drive that came with your computer.

https://www.amazon.com/SDTC-Tech-Splitter-90-Degree-Extension/dp/B07NYZZDZR/

Extenders with right angle cables are not expensive.

I should have said the SATA data cable should have a right-angle connector. There should be spare SATA power connectors available that already have right-angled connectors.

The same is true for the new XPS 8940.  Four empty bays (two 2.5 and two 3.5 bays).  The $2700 computer comes with one ONE 2.5 plastic rail.  The rails seem to be specific to the chassis, so good luck finding one even on the Dell site.

Dell touts this new chassis as tool-less...unless you need to install a 3.5" drive.  Then you'll need a Phillips screwdriver and a diagram (on a sticker on the lid) to know how to remove the steel cage properly.

Also, the 8940 only has three available data cables (two near the 2.5 bays and one near the 3.5).  So much for loading it up with four drives.

According to the XPS 8940 service manual, the computer comes with two 2.5" drive cages.

In a discussion with Technical Support, I have been advised that the computer no longer comes withe the 2.5" drive cages.

Tech Support advised that the part number for the blue plastic 2.5" drive cage is: "YHNFX". 

Do you mean blue plastic hard drive carrier not "blue plastic 2.5" drive cage"?