No there is no second mount. yes machine supports multiple HDDs but has only one mounting bracket. See below. The highlighted HDD is the only mounting available inside the case [IMAGE]         No there is no second mount, you can either mount a 3.5 inch or 2.5 inch drive. If you bought the computer only with one SSD (NVME) HDD mount will not be included and needs to purchase separately.    Yes You can swap the cable that way or you can simply clone your SSD, boot from it and Format the HD. this way no cable swapping will be needed. However I see no need of doing it.  1. You main OS drive (C drive) should be the NVME SSD which plugs directly to the motherboard without the need of any cables.  which is the fastest out of the three.  2. I believe you already have a second drive (3.5 inch HD)  3. The third drive can use an available SATA connector from motherboard. However it will be lying somewhere inside chassis as there is no mounting available for it. In case you opt for a 2.5 inch SSD, you can tape it some where below ODD bracket. I have been doing with a different model, since it's a desktop it hardly moves any where on the table.    I think the purpose of limiting HDD bracket it convince users to buy XPS series which comes with 3 brackets  
According to the Setup and Specifications Manual, the maximum HD size is 2TB for my Inspiron 3910. Q1:  Is that actually true?  If so, why?   Also, according to the Setup and Specifications Manual, my Inspiron 3910 will support up to  One 2.5-inch or 3.5-inch hard drive + one M.2 2230/2280 solid-state drive (which is what came in the machine - along with an optical drive). The manual also indicates that there are three SATA 3.0 slots.  I could not find a picture of the MB to reference, but I did eyeball it and there does seem to be three (3) SATA connectors.  It looks like one of the connectors is attached to the HD and the second connector is attached to the optical drive. Q2:  If there is an extra available SATA connector, can I not add a third (technically 4th if you count the optical drive) storage device?  Or is there something funky going on that renders that extra SATA connector unusable? Q2B:  If the SATA connector is usable, what is your suggestion for how to physically mount the extra drive?  There does not seem to be any information in the manual for attaching additional storage and the giant plastic CPU airflow device is consuming most of the available space inside the case. Q2C:  If the extra SATA connector is not usable for some reason, could I get a PCIe x1 Slot SATA expansion card, plug the optical drive into that and use the MB SATA connector (that the optical drive was formerly plugged in to) for an additional SSD/HD?  If so, same follow-up question as Q2B - how do I mount the new drive in the case?  
Inspiron 3910 - Storage Options: Is it possible to add a third SSD or HD