@Sifu1961  

The 3.5" HDD caddies and 2.5" HDD caddies are interchangeable. You can have up three 3.5" HDDs in the chasses, though that would add a lot of heat.  You can buy another caddy on eBay. 

AFTER

Yes.  You just have to lift up the LED wire in the back as the HDD caddy will go all the way in. 

"The same dell support agent said the max tb fir each drive is 2tb, hope that's right"

There is no 2TB limit to a storage drive.  You can put a 6 TB storage drive in there if you want. 

The 2TB issue only arises for a boot drive partition, if the boot drive is MBR.  

So i can install a 3.5 caddy where a 2.5 caddy was?

According to the specs here you can have 2 x 2.5" drives and a single 3.5" drive. The 2 drive cages you see are for 2.5" drives. The 3.5" drive location is behind the PSU as shown in this picture on the support page. And on this pdf of the same pages (25-27) which is a bit clearer.

@Sifu1961 

Do a search google or eBay or whatnot for Dell part number "13P1-4YN0C01" 

There are a number of options under $10 free shipping for the 3.5" HDD caddy. They 3.5" HDD caddy dimensions are the same from the R5 to R11. 

Thanks for the clarification,  looks like I'll have to swap out the 1 tb for the 2tb and buy another 2tb ssd 2.5. The same dell support agent said the max tb fir each drive is 2tb, hope that's right

Think I'll forego that option, too green to mive wires and pop in caddy extensions,  just hoping i hooked the one i put in correctly to an existing cable with 2 connectors. Thanks again for the help

Crud, they even showed me the spec sheet with the maximum tb limit, otherwise i would have gotten a 4 or 6 tb ssd. Oh well, hey thanks for the help.

I want 3.5" drives in those bottom bays - is there actually a caddy for it?  I can't find them...

"You can buy another caddy on eBay. "

can you point me to where because the upper bay caddy and two lower bays are NOT interchangeable by any means.

always the question - but I don't see the mechanicals available to do this.  There's barely enough clearance front to back for a 3.5" drive and with SATA and power, it didn't fit....when I tried it based on these claims.

try searching for 13P1-4YN0G01, found one on ebay.

Alienware Dell Aurora R5 3.5" HDD Hard Drive Caddy Tray Bracket 13P1-4YN0G01 | eBay

It doesn't list the R10 as a compatible model with that part.

The drive on the top right is ready made for a 3.5" drive. Its also the most convenient slot since it has both the data and power cable provided. There is no need to buy and convert the 2.5" slots.