I just bought a used Lenovo ThinkCentre M710Q Mini Tiny Desktop PC Computer i5 6400T 1TB SSD Win 10 Pro from Ebay for $289 AUD and plugged in some oldish external SSDs and HDDs and now have 10TB of storage. I’m really pleased with it, it took about half an hour to install Proxmox and I’ve now got 5 VMs up and running.

I wouldn’t recommend a WD My Cloud Home - it’s not a NAS as such, it’s a bit limited; I’d go for a Synology. or One Drive as you suggest - a 1TB plan is quite reasonable with regards to cost.

Thanks for the responses; it seems I can’t really do it. I looked into ZFS but if I use that it halves the available disk space to 2TB. I’m using the VM for a media server and thought it would be better to have 1 4TB space instead of 2 2TB disks. At the end of the day it isn’t a big deal, I just thought I’d be able to present both disks as 1.

I use uptime kuma for monitoring - really easy to set up and very versatile