How to access both drives

On my MyCloud Dashboard Home it only shows 4TB Drive and it is full. But on Storage RAID it also only shows a 4TB Drive.

But on Storage Disk Status it shows 2 System disks each are 4TB

How do I get to use Disk2?

I changed the RAID setting to Spanning. I Hope that was the right thing to do

Well, if you want maximum space, yes Spanning or JBOD is good. But if one of the drive fail in Spanning mode then you will lose all your data. RAID 1 mode in the other hand you will get half of the disk space but you gain of still having your data incase one of the hard drive fail. RAID 1 also let you rebuild when you replace one of the bad hard drive.