R730xd not seeing the rear drives

I recently purchased a used R730xd, and I can't get it to recognize the two SSDs that I've put into the rear 2.5" drive bays.

I have the 12x3.5" system for the front, but I don't have any drives in that yet. Just the two in the back.

This system has the PERC H730 mini.

Looking at https://dell.to/45QgGQ7, my system is the figure 6 way (why are the parts numbered in the image, but the legend uses letters???)

I have checked the cables - I have even unplugged the cables (both ends) and plugged them back in.

In IDRAC, I'm getting the message:

RAC0501: There are no physical disks to be displayed. 1. Check if the host system is powered off or shutdown. 2. Check if the physical disks are inserted into the enclosure or attached to the backplane. 3. There are no out-of-band capable controllers detected.

In searching the internet for assistance, I've seen posts to check bios (I've gone through every bios page - the rear backplane is enabled, not sure if there is anything else to check), cables, and . I'm not sure what else I need to do to get this working.

In the Dell Lifecycle Controller, when I go to physical disks, I'm told that there are no disks.

I've run hardware diagnostics, but it skips storage since it doesn't see any drives.

Both of the drives appear to be fully seated - but how do I ensure that they are?

I do not have any lights on the drives.

The system has two CPUs.
I have updated (via IDRAC) all of the firmware.

Once I get some screws to mount HDDs in the front bays, I'll be able to tell if those are detected (used system... came without those screws... arg!!!).

So, what am I missing to get this working?

Thanks for your assistance!