790 models use R494D INSIDE the 3.5 adapter.

https://www.youtube.com/watch?v=ilaG2Lx1Oi4

https://www.youtube.com/watch?v=6NBaAvoUt0A

It supports 2 Drives whether SSD or HDD as long as they are 2.5 inch.

The catch is that you need sata power splitter and another sata cable.

DELL N701D cable.

https://www.amazon.com/Genuine-Dell-FOXCONN-SPLITTER-Desktop/dp/B00T74H5OE

  A Silverstone adapter has all the mount points of a standard 3.5" hard drive, and it's only $12 with free shipping. It holds two 2.5" devices stacked.

I know about the various adapters that allow 2.5 inch drives to be installed in a 3.5 inch bay!

I'm talking about 2.5 inch bays

Thanks anyway. Maybe someone else can help.

R494D fits inside the 3.5 bay which is common for these models. 390 790 990 MT.  There are no 2.5 bays.  End of story.

There are no 2.5 bays.  You have to use the official R494D or Silverstone Adapter or something else.  Nobody else can help with something that does not exist for this model.

You can also put four in the 5.25 inch bay.

https://www.amazon.com/ICY-DOCK-Rugged-Backplane-External/dp/B078QDHHQG/

https://www.amazon.com/SilverStone-SDP09-2-5-Inch-3-5-Inch-Hot-Swappable/dp/B0049MPQDG

Added advantage of R494D is that it supports 2x 2.5 drives in the 3.5 bay.

https://www.youtube.com/watch?v=wlB7d7nr1Jo

ebay sellers sell endless caddies and adapters,  ever look there,  heck you can tie warp the 2.5 in down, your sure can, it has no motor, or head to crash it is SOLID STATE AND DOES NOT VIBRATE LIKE HDD DO.

just do whatever and be happy.

even tired down

or with cheap 3d PRINTED CADDIES FOR A BUCK

OR PUT IN GUTTED FLOOPY

OR PUT IT ON THE DVD SPOT USING A NEW BRACKET DIRT CHEAP ,( MOST FOLKS NOT ME HAVE NO USE FOR DVD) SO REMOVE IT.

The sell 2.5 drive cages generic,  to be mounted where ever you want.

try searching on ebay and learn what is sold there,  it's endless there, IMO !

I even have dell servers with the mini dvd gutted and boots to ssd, and now have 8 RAID array, yah, raid 6.

this is no hard at all pure physical  mounting and vast vast methods, if you ask can show them all.

done them all I have, and not  one ever failed,

@speedstep 

The 790 owners manual shows them on pages 85-86.

It's under the specifications lists. It shows 5.25 external accessible and internal bays, both 3.5 and 2.5 inch. MT is supposed to have two of the latter.

2.5–inch SATA drive bays
Mini-Tower two
Desktop one
Small Form Factor one
Ultra Small Form Factor one

Hi @donm4599x,

Could you upload photo of your 790 MT's HDD bays?

ALL Models of 360 760 960   390 790 990 use 3.5 drive caddies.

If 2.5 drives are there they are mounted in R494D then snapped into the 3.5 drive caddy..

https://www.youtube.com/watch?v=6NBaAvoUt0A

https://www.youtube.com/watch?v=wlB7d7nr1Jo

The caddy for Desktop vs Mini tower is different but the R494D is the same in both.  2.5 specific bays do not exist as caddys.

https://www.dell.com/support/home/us/en/04/product-support/product/optiplex-790/docs

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-790_owner%27s%20manual_en-us.pdf

3.5 and 2.5 and Sata Power Splitter

https://www.amazon.com/Dell-Desktop-R494D-Drive-Optiplex/dp/B00XCGQLZC

ALL 790 BAYS are 3.5 R494D 2.5 inch caddy fits inside the DN8MY 3.5 inch blue caddy

https://www.amazon.com/dp/B07NNQ5C4M/

The optical bays are 5.25 inch.

 optiplex-790_owner's manual2_en-us.pdf

I don't need to read the spec.  I own both Tower and Desktop and even SFF models.

The 2.5 bays are R494D INSIDE the 3.5 caddy.

There are not Specific 2.5 bays like the Inspiron 3650 or 3670 have.

Believe whatever you want.

You are asking for something that does not exist.  (2.5 caddy and bay in ANY 390,790,990, 3010 7010 9010, 3020 7020 9020.

DELL N701D sata power splitter is needed to use 2x 2.5 drives in the R494D

MT, Desktop, SFF doesnt matter. They all have 3.5 caddy.

Thats why I have the youtube videos that show the R494D inside the 3.5 caddy.  I also posted pictures of exactly what I am talking about.  End of story.

The 780 is also the same.

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-780_service%20manual2_en-us.pdf

The 2 bays in the tower and the 5.25 inch Optical bays are the same.

Between 780 and 790 the covers changed but the inside did not with the exception that the floppy drive controller and cables from 745 755 760 960 are no longer in the 780 790 990 9010 9020.

R494D supports 2 Drives whether SSD or not as long as they are 2.5 inch.

The catch is that you need sata power splitter and another sata cable.

https://www.ebay.com/itm/Dell-980-3010-7020-T1700-T5610-T3610-T5600-2-5-SSD-Hard-Drive-Caddy-R494D-J132D/233439581840

DELL N701D cable.

https://www.amazon.com/Genuine-Dell-FOXCONN-SPLITTER-Desktop/dp/B00T74H5OE

@speedstep 

Glad you found the manual. Please read the specs on page 85-86.

Thanks.

@bradthetechnut 

I'm trying to find a picture myself! There is a sheet metal structure at about the middle of the chassis, just behind the front panel. It would be about where the dell logo is on the front panel, and above the 3.5" disk drive cage. I don't know what it is for, but thought that might be the place. It was definitely made for something, but who knows. Maybe floppy drives, but you'd have to saw out the front panel for that 

The only thing that indicates any 2.5 inch bays is in the user manual I referenced, in the specifications section. But there is no other mention of these bays, such as instructions/pictures for installing/removing the drives as there is for the 3.5 drives and optical drives.

Edit: Maybe this structure is a vestige of where floppies were mounted in earlier models? Maybe they used the older model cases except a new front panel?

Edit 2: Looking at pics in the user's manual, I may be remembering the bracket structure just below the optical drive bay ... it is right behind the front panel section where the usb and audio sockets are located. What was that structure used for?

Using the r494d parts, there would be Four 2.5 inch bays (since there are two 3.5 bays). The dell spec I referenced says two 2.5 bays, so it still doesn't make sense.

Did they take page 85 and 86 out of the document? The pdf I have was downloaded a year ago, so maybe they have edited it since then.

It just so happens I have a photo of the inside of a 790 MT.  There's no 2.5" bays.

You must have a different owner's manual.  Mine doesn't mention HDD bays on pgs. 85-86.

Optiplex 790 Tech Guide 

EDIT:  I erred.  The images above are from the 790 DT manual.  However, I can't delete images.

@bradthetechnut 

I think it was actually the floppy bay on earlier models. I looked at a 780MT service manual, and it had a nice picture of removing the floppy, which was immediately below the 5.25 inch bays. There's probably enough room for a couple of 2.5" devices if you are handy making sheet metal parts or just glop some type of putty in there 

In the manual I downloaded last year  the drives are mentioned in the specifications section, near the end of the manual. Now, I'm thinking it was just dell's way of saying " you can have two 2.5 inch drives, if you buy an adapter such as r494d and cram it into a 3.5 inch bay "

I just downloaded the manual again from here:

https://www.dell.com/support/home/us/en/04/product-support/product/optiplex-790/docs

The link is a little ways down ...

OptiPlex 790 Owner's Manual (Mini-Tower)

Anyway, many thanks for the replies, and @speedstep ,sorry to have bothered you; I know you are very busy.

If you're thinking of the structure (or absence of) between the 5.25" bays and the HDD bays, it's used for nothing.  Except for what I used it for on my 7010, I super glued a blue led fan there.