I found it confusing that I still find information about a VM (also on Nextcloudpi.com), but not a download link for that anymore. This post here talks about using an LXD like a LXC and that this is not really working, while I also read in some posts here that people seem to have managed that. So this is overall not clear, what is outdated and what the recent status quo is.
Nevertheless, I got a CT working with the help of Proxmox Helper Scripts | Proxmox Scripts For Home Automation
So this working.
I think I can also achieve to get the HDD into the CT at all.
The topic is more what happens then to bring my old HDD together with a datadir path that I cannot edit via ncp-config and that points to a directory that I probably need to adjust.
But this is then more related to How to configure an external USB drive with NextCloudPi - #7 by johndoe0815 where you also started replying, while this thread here is more on how to bring the LXD into Proxmox.