NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 8G 0 disk
├─sda1 8:1 0 487M 0 part /boot
├─sda2 8:2 0 1K 0 part
└─sda5 8:5 0 7.5G 0 part
├─nextcloud--vg-root 252:0 0 5.5G 0 lvm /
└─nextcloud--vg-swap_1 252:1 0 2G 0 lvm [SWAP]
sdb 8:16 0 500G 0 disk
sr0 11:0 1 1024M 0 rom
sr1 11:1 1 1024M 0 rom

oguruma .. I admit to being at a bit of a loss here ... You mentioned in your first post that your system had a 120GB SSD drive and a 2 TB secondary drive. You wanted to use the 2 TB drive for various needs, including as a backup destination and to hold a VM you wished to set up to run a web server (nextcloud).

What I am seeing above are one physical 8 GB drive (?) divided into 3 partitions, the largest (7.5 GB) of which is divided into 2 volume groups of 5.5 GB (root) and 2.5GB (swap), and another physical 500GB drive, currently with no partitions at all.

If I am reading this correctly, please explain, otherwise ... please explain.

Others here (michaelk, habitual ... ? ) ... please chime in if I'm missing something obvious and you feel you can better help our friend oguruma ...

It would have to be obvious for me to see it.

I'm not sure the OP isn't "tripping over a dollar to pick up a nickle."
3 pages in and key details are just showing up. Yak shaving is not my thing.

There was a bit of an additional learning curve due to virtual machines.

sda1 is /boot
sda2 is an extended partition that contains sda5 which is a LVM
The LVM contains a root and swap VG.
sdb is additional virtual disk and has not been partitioned yet.

The OP wanted an additional 500GB to use as a data drive. You can use gparted to create a partition table, partitions and then format as desired. You can mount it where ever and add it to the /etc/fstab just like you did with the 2TB physical disk. If you want to add space to your existing /(root) that requires some additional steps.

I recently 'went' LVM myself and to be honest, this would fsck me up too.
I rely on "If it's hard, I'm doing it wrong.". Can't speak to any one else.

Peace!

... Is all of this WITHIN the VM ? Otherwise, I'm still lost and of absolutely NO use to the OP ... :-( ... Even if it is within the VM, I can't for the life of me figure out what the strategy is there ...

... I apologize, oguruma ... I think I'll have to bow out here. I don't think I've been much help up until now and I'm not sure I'll be very useful going forward .. I'm rather lost with respect to how you seem to have set things up. I'll leave you in the more capable hands of some of the other fine folks here ...

Best !

This "situation" is a little kludgy IMO, and my suspicion is that is it implemented poorly.

Users are now trying to take things to a whole new level and this can only be good for the product or service.

Or Skill.
People have to learn somewhere and I can appreciate that.

I like the K.I.S.S. method because I over think everything, and just because I'm complicated, doesn't mean Linux
has to be. IMO.

No offense to the OP. I hope.
Good Luck.