ioscan will tell you what disks are connected and diskinfo what they are..
what do you call disk usage? Are you in LVM?
If so are you wondering if you have some spare unused lv extents because there is difference between disk size and what you see with bdf?
Many cases:
1 - Swap: will not show up with bdf, df, du, but with swapinfo
2 - Unmounted FS or unused FS...
3 - Mirroring...
4 - Yes not all disk space is used
in this case free PP can be seen with vgdisplay /dev/<VGname>
man vgdisplay lvdisplay pvdisplay swapinfo