In the VM i formatted a virtual 8GB hard drive for OS and the result is this. I have the same situation in my actual OMV4 server: 8GB usb stick, 4gb of usable space.
I think is better to close this thread, it's becoming a mess of random question...
Now that i killed my hardware, and i'm waiting (hopefully) for the new components to arrive, i need to study better and try to understand the basis of the configuration. Probably is better to open new thread(s) when i'll build the new system (or when i'll play in a VM)
I'll need to configure and use containers, i'll need to transfer my hard drive, i'll need to configure multiple drives... a lot of thing to do (at least for me).
Thanks for all the replies, OMV is a great OS with a great community!
It's probably taking up half the drive for swap. It's partly why I think 8gigs is to small a recommendation for minimum installs, as you can see. 16-32gigs is much more realistic.
It's not really a mess of random questions at this point as it's common install issues (your other questions may well be more appropriate for new threads).
1. Your install issue is common. Either when folks have multiple drives hooked up during install... or in a small amount of cases (with the OMV installer) the installer flash drive gets picked up as /sda, instead of /sdb. This is where the debian installer helps, as at the end, you can choose where you want root installed
2. It's just an issue of your OS drive being to small.
Testing in Virtualbox is a great idea (I still do it a lot).