Cable matters! So does size...

Op want to install 5.25 hh bd in external hdd flex bay. That bay is designed for external 3.5” removable hdd so should have regular sata power and data cable. Seems op pc is missing the Dell oem sata power cable for external hdd which connects to motherboard 

it is corrrect that cable mini atx pinout is proprietary and not c/w precision motherboard 

Does op original slim odd SATA power cable have a Y connector end that is regular sata size?

https://www.dell.com/community/en/conversations/dell-pro-desktops/fct2250-cables/6944af89de43d31695626ea7

https://www.ebay.com/itm/397349998243

Since op was testing slim odd from precision the cable used was probably from precision too and the correct fct2250 Y hdd odd cable is not present. Ask seller to send high res pic of hdd connector to confirm it is regular sata power for hdd which is c/w 5.25 bd

Thank you, @redxps630 . The system came with these two cables CBL, PWR, 1ST/2ND HDD, D14-CP5H9 and  CBL, Y, HDD/ODD, PWR, FRT, D14 -34MT4 (x2 for the CP5H9) -so I have one HDD and one SATA SSD hooked up to those and x1 of the Y cable, and so I have a 9.5 mm ODD -the LG BU 40N hooked up to the SATA ODD and there is a 4 pin ATX like connector free on the Y cable which I assume is if I want to hook up a flex bay SATA SSD or a HDD into the Flex bay which sits above the ODD bay-on the Precision 3680 this end of the Y cable has a regular size SATA connector, to which one can attach any HDD or a 5.25"" Disk Drive but not on the Y cable of the Pro Max-its more proprietary it appears! I checked the eBay link (thanks). There is only 1 blurry picture, so I cannot be sure, but that looks like the Y cable I already have: the 34MT4. I asked him for a high-res pic, but based on the part number in his description, I suspect this is the same Y cable that already came with the system. Thank you for the input. Open to further help.

Got it. Since The system came with these two cables CBL, PWR, 1ST/2ND HDD, D14-CP5H9

can you use a long enough sata power splitter to make a third regular hdd sata power connector for 5.25 bd?  The idea is the splitter can let you power both ssd and bd assuming ssd takes little power and the Dell cable was spec’d to support power of 3.5 hdd comparable to bd

https://www.amazon.com/Zheino-Splitter-6-Inch-Y-Splitter-Extension/dp/B01MZBDDPM

@redxps630​ -Thank you! That's a great idea! It will need a long cable (like the one on the Precision 3680, in which they had done that-the cable connected to the ATX pinout on the MB had 4 connectors: one for a small ODD, and 3 other regular SATA connectors - 1 for a Flexbay HDD or SATA SSD, and then 2 more for 2 other HDDs or SSDs in regular bays-1 of which bay was at the other corner of the case!).

I see only one 8 pin mini ATX sata pwr socket on fct2250 board. How does Dell let user connect both cables below to that socket?  Can you connect only one of the two cables ?

@redxps630​ - the ATX pin out of the motherboard has the cable that connects to the ODD and the flex-bay SATA HDD or SSD (even here that cable is very proprietary in that the connection for the HDD/SSD does not have a SATA end but has a 4 pin or 6 pin ATX female end!). Then for the other 2 SATA HDD/SSD which are on the opposite side of the case there is a funny looking connector coming out of the side of the mother board and the cable connected to this splits into two with the usual SATA connector at the end. That SATA power connector is again proprietary I think! 

@redxps630 -here’s the MB with the connections circled in red:

I see there is a subtle 90 degree SATA PWR at the edge of board which Dell user manual does not document.  sloppy documentation there.

I see the flex bay may have a regular sata pwr and data socket screwed on a metal piece like this, which would allow a 3.5 hdd to fit in.  If you have this, you may remove the fastening screw of that metal piece and use those sockets for a 5.25 bd.  The metal pieces is designed to be in a fixed position at back of flex bay for swap of hdd, which is at a different position of the connector for 5.25 bd I think.

@redxps630​ Those are good pictures. Here's the twist: The flex bay connector on the Precision 3680 appears to have a different back portion cable. The portion that comes out of that flex bay back is a regular SATA 'hangy thingy' 'female' end which then plugs into the 'male' end of the SATA cable header, which is one of 4 cable heads coming out of the cable that plugs into an ATX connector on the Precision's MB and that ATX cable does fit into the Pro Max MB's SATA power ATX pinout but did not seem to power the Drive LOL! When I examine the ends of the ATX connection pinout, the 'dummied holes' -the ones that don't seem to have metal lining them- seem to be different on them, perhaps because they are wired differently for each MB (?). I don't know if I make sense. I'll try to get pictures once I crack open both cases again. Anyway, it's all a bit murky. I think I'll just try to run a cable from the SATA cable going into the HDD1 or 2.