Is this you model? https://www.dell.com/support/manuals/en-us/dell-ect1250-desktop/dell-tower-ect1250-owners-manual/major-components-of-dell-tower-ect1250?guid=guid-54f725c8-85e9-4226-9a79-4e262cf9bbc0&lang=en-us

If so, look at the drawing for where the hard disk is, and the drawing is 'exploded' to show all the component.  
#17 is the 'holder' for a hard drive, and it probably connects to the back top of the case, you can see one slot there. There are 2 cables below the drive and are the ones you want. Possible the drive holder could connect to the side panel (shown on top #1).

Quite possible that the disk holder and cables are only in a unit that shipped with a hard drive. If so, I suspect?

However, IF this is your model, PARTS does NOT have what I think you need  either --> https://www.dell.com/en-us/shop/dell-hdd-rotation-bay-with-cable-screw-for-dell-pro-tower-tower-plus-desktop/apd/575-bctb/storage-drives-media ?

However, check this out ---> https://www.dell.com/en-us/shop/dell-hdd-rotation-bay-with-cable-screw-for-dell-pro-tower-tower-plus-desktop/apd/575-bctb/storage-drives-media and it DOES say it is for the QCT1250???

I would suggest CALLING Parts to verify if it IS really for After Market? It might REQUIRE a 'different' case to have it attach too?

This Dell was purchased directly from Dell. My QCT1250 doesn't look like what you sent me. My Tower has no drive bays for Hard Drives at all. They do have several spots for M.2 slots. But the MB does have 2 SATA ports but no drive bays.

@brad-bradheslop.com​ 

Hmm, that IS the manual for the OCT1250's link? Does the front plate look the same?

To be honest, #20 is an Optical Drive. I DID NOT see how you could access it though? No 'hole' in the front case?

Again, maybe a call to PARTS would help here?

@DELL-ChrisM2 is the manual accurate? Are the Parts I lists actually for this PC?

Thanks, the new line of Dell's is very confusing. Increasing the pricing and cutting vital equipment out. Need the OptiPlex back. Dell needs to be more honest.

Plus Dell has a no return policy that is only showing in the Sales Rep e-mail signature.

The legal warranty and forms don't show a no return policy.

It depends on whether the system was ordered by a private individual (hence consumer sale), where a 30-day return applies or by a business (where the return period doesn't apply).

the new Dell tower and SFF case are phasing out hdd and odd in favor of m.2 ssd as main storage offering.  the traditional 3.5 hdd and odd can be shipped if ordered which comes in a hard drive bay.  if ssd is the only ordered storage. that hdd bay is not shipped.  hence OP found no place in case to mount the hdd.

https://www.dell.com/support/contents/en-us/videos/videoplayer/how-to-install-the-hard-drive-optical-drive-and-drive-bay/6377058990112

https://www.dell.com/community/en/conversations/optiplex-desktops/hard-drive-bay-not-shipped-if-pc-shipped-with-ssd-only/691dc0e401c94d3edab3078c

see this extensive thread too

https://www.dell.com/community/en/conversations/dell-pro-desktops/tower-ect1250-help-adding-extra-hard-drive-need-satapwr-cable/680cbfbecf0d62741ca491c5

Dell hides the spec detail in manual

in QCT1250

in QCS1250

remove hdd bay technical procedure, but not mentioned in Specification.  Dell manual documentation is disorganized.

NOTE:The drive bay is only available in computers shipped with a hard drive or an optical drive.

NOTE:The drive bay is only available in computers shipped with a hard drive or an optical drive.

https://www.dell.com/support/manuals/en-us/dell-pro-qct1250-desktop/dell-pro-tower-qct1250-owners-manual/removing-the-drive-bay?guid=guid-8b4536fc-0dfb-48c1-b74f-35337273cfcd&lang=en-us

https://www.dell.com/support/manuals/en-us/dell-pro-qcs1250-slim-desktop/dell-pro-slim-qcs1250_om/removing-the-drive-bay?guid=guid-3f78b0cc-8397-4879-9182-96ed54b0dccd&lang=en-us

I will have to give Dell credit though, it is so much easier to find the link to Dell manual (despite its internal inelegance) compared to HP, which is 100 times worse in archiving desktop manual/spec.

@redxps630​ 

So why then wouldn't this PART (https://www.dell.com/en-us/shop/dell-hdd-rotation-bay-with-cable-screw-for-dell-pro-tower-tower-plus-desktop/apd/575-bctb/storage-drives-media) as it specifically calls out the QCT1250 in the page?

Dell parts website compatibility documentation is historically unreliable.  It can omit compatible models and include incompatible ones. Seen it many times before. Buyer has to do research to confirm it is a compatible part, in other words do own homework.

This official Dell video tilted “How to replace the 3.5 inch HDD on Dell Tower ECT1250/Dell Slim ECS1250”

@redxps630​ 

OP has an QCT model, is the case the same as the ECT?

yes QCT and ECT are same metal chassis with different motherboard.  Dell does this all the time across model lines of same generation.

I want the OptiPlex Desktops back. The new Dell product line stinks. I just purchased a Dell Pro Tower QCT1250. More expensive and very cheap parts. No place to put the internal hard drive. Have to tape it to the bottom. No power cables available on Amazon that will work. Why should I have to go directly to Dell to get a SATA power cable? Power supplies are very limited on  the new desktops. The drive bay is an add-on. Really? I would think a Desktop name with the word PRO in it wouldn't be a starter and useless computer. 

You have learned the Pro in model is marketing name.  Like Windows Pro vs Home edition, Pro desktop has more (optional) features but that does not necessarily mean it has to be plug n play to support traditional 3.5 hdd.  HDD is being phased out and Dell might as well include it in Home Desktop model line only if there will be such line.