Digging into the FAQ, it looks as though they might use ECT1250 and ECS1250 chassis, which both feature a single 3.5" drive bay.

@asbatwrk​  My ECS1250 Slim has no SATA 3.5" drive bay. Dell cannot supply it as a spare part, despite marketing the machine as upgradable to HDD/SSD.

Dell upgrade capability assurances:

Dell Pro Slim (FCS1250) do have a single 3.5" drive bay (sort of) but you CANNOT fit a drive into it without special Dell fittings which are NOT supplied unless you order a hard drive at the same time as ordering the PC!!  In which case you would know that a hard drive can be fitted!

How do I know you ask?  Well this is exactly what has happened to me, see my rant over here:

https://www.dell.com/community/en/conversations/dell-pro-desktops/hard-disk-in-dell-pro-max-slim-is-impossible/68892d8277e0591bad919b66

They do fit a hard disk power lead (but it is difficult to find) it is tucked away by a clip near the rear fan and they don't supply an ATA cable - not really a problem.

You can see the HDD plastic bushes/Screws required in the FCS1250 online manual under the section "removing and install customer replaceable units" > "Hard Drive" either install or remove show them.  It is best to use the online/HTTP version because of the two PDF versions that are pointed at from the manuals section of the FCS1250 one the PDFs is for a laptop not the desktop - so doesn't really help - the oneline ones are correct.

Regards

Callum

@KingDondee​ Hi,  the ECS1250 manual appears to suggest that you can install in the "rotation bay" - which is NOT obvious to find (it also might be difficult to find an ATA power supply but I suspect it will be there if tucked away somewhere) - using standard HDD mounting screws, see:

https://www.dell.com/support/manuals/en-uk/dell-ecs1250-slim-desktop/dell-slim-ecs1250-owners-manual/rotation-bay?guid=guid-da4de038-b95c-42f7-b11a-2e1baf8bd6c0&lang=en-us

and

https://www.dell.com/support/manuals/en-uk/dell-ecs1250-slim-desktop/dell-slim-ecs1250-owners-manual/removing-the-hard-drive?guid=guid-479abae4-8266-48b5-9e95-9b163f3eb4a2&lang=en-us

Unlike my rant about the FCS1250 which requires special plastic bushed screws for it to work, see here:

https://www.dell.com/community/en/conversations/dell-pro-desktops/hard-disk-in-dell-pro-max-slim-is-impossible/68892d8277e0591bad919b66

Regards

Callum

Found the below after reading your posts as I have the same issue, will order and let you know if it does the job....

kenable PCI Backplane Adapter for Mounting 2.5 SSD/HDDs Into Vacant PCI Slot

https://www.amazon.co.uk/dp/B01GQHNQVK?psc=1&ref_=cm_sw_r_cp_ud_ct_PCGTTSXJMYFQBQWWNKE7_1

@almcgd​ Hi. Yes I bought similar. My compact Dell Slim ECS1250 requires a hinged mount for a SATA drive as it otherwise blocks access to M/B. Luckily my purchased generic mount has two std screw holes that match the pre-drill for a hinged mount. I can cope. 

The bigger issue is still the pinout voltages on the mini-6pin SATA power cable required. Generic could be fatal. I have finally obtained a part no. & ordered from Dell Australia. Hopefully correct. See below. I will post outcome after delivery & install.

Supposed to be power cable for Dell laptop adaptable to Dell Slim ECS1250 for SATA drive. Let's see.

The SATA to 6-pin power cable supplied as above fits perfectly. I have not yet energised the SSD but feel sure it will run & be recognized.