Disk Partitions Not Listed in Device Manager

I’ve poured over the documentation & FAQ’s and have not been able to get beyond this obstacle.
I did a fresh install of R.4.1.0, preserving important pre-existing ext4 partitions, just as I did in R.4.0.1. (During disk config, I set mount points & labeled these partitions w/o a reformat.)
In this latest iteration, I find that the disks are listed in a Dom0 Terminal as labeled, and are shown mounted as /dev/sda[n] w/ dom0:fdisk. However, they are not present in Device Manager; I am unable to attach any of these partitions to my user qubes.
Before I realized their absence in Device Manager, I tried in dom0 terminal:

qvm-block attach dom0:sda[n]

And of course I got:

qvm-block: error: backend vm ‘dom0’ doesn’t expose device ‘sda[n]’

Where am I in error, what am I missing, or need to do to get Device Manager to recognize my pre-existing partitions?
What other information might be required for someone to assist me in resolving this situation?
Is this even the right venue?