I have also got a dell optiplex and Proxmox and a Debian machine with mostly Arr in containers.

I also have 3 old HDDs that I'm using for it, of various sizes (2x1tb and 3/4tb)

I have used a powered usb3 to sata connector to hook them up to the Dell. As in I have 3 usb to sata connectors, and each one also has a plug socket. I've got those 3 plus my Dell in a 4 gang extension lead so it's only using 1 plug socket in my kitchen. They were about £15 each on Amazon.

I have had a gotcha from using usb that destroyed my setup.

I now have a working solution that will stop this happening again:

So I have 3 usb-sata HDDs hooked in via the usb ports. I set up Open Media Vault as a VM. Then I passed through the USB drives to the OMV VM.

Those drives are then shared via SMB (I've just added NFS) to everything that needs it via the OMV VM.

I can then access the HDDs via this SMB share in Proxmox for backups if that's what you wanna do.

Now the reason I've done it this way is because originally I had the drives in Proxmox. I gave them names and then put those names in the Fstab of Proxmox. One of the drives "forgot" it's name and Proxmox wouldn't then boot because one of its drives wasn't accessible.

You could get around this by adding an option to the end of your fstab, I think the option is "nofail" but I'm not 100% so just check up on it.

I've used this option in my Debian VM Fstab to mount the NAS drives so my Arr stack can see them, and even got my Squeezebox server using them too. I'm using CIFS so I can see the drives on my Windows PC so I can manage the storage on my desktop.

So there you go, usb HDDs with passed through usb sockets to OMV VM is how I do it. If one of the drives fails OMV still runs and I just find I have no media in Plex and have to figure out why my drive isn't working anymore.