thanks for the screenshots jose,
(if you are able to easily capture the error message that dashboard gives you, that could be useful too)
It seems as though you are using Dual Drive Redundancy? (ddr), and if i go by the screenshot it seems as follows:
8 drives x1TB each = 8TB
8TB -2TB (2 drives for ddr) = 6TB
6TB x0.9 (for filesystem/overhead) = about 5.4TB available for use.
(this would match up with your screenshot of dashboard, and as it looks as though your max volume size is set to 4TB, then i would also expect a 2nd volume to be available for the remaining 1.4TB (or thereabouts)
as you mentioned that you definitely had a 2nd volume before… is there any more information that you can remember about it (such as if it worked fine before, and how long you had a working 2nd volume for, and if anything had changed since then, such as downgrading a drive or something else? (and did you happen to use single SDR before and then later switched into DDR?)
also, were you possibly using usb before when you first setup that 2nd volume?
there is an article here that may help, which suggests using usb (in order to carry out the setup of the 2nd volume via dashboard and Volume Management facility)
(however, if your current iscsi works, it might be best to try usb and not to upgrade dashboard in case iscsi stops working which may possibly require a lot of iscsi shenanigans to correct)