I can think of two possibility or workaround here,
1> Remove the un-used SSD half of the slot and then try to create the Virtual DIsk Group with SAS drives as you want in Pool A. It's possible that Controller is thinking SSD drive available in system without Performance Tier License so it's not allowing to create new VDG. So after you create the new VDG in Pool A then insert the SSD and try to configure SSD read-cache
2> Delete the existing SSD read-cache VDG from Pool B and then remove all SSD drives half of each slot. Then try to create the Virtual DIsk Group with SAS drives as you want in Pool A. After you create the new VDG in Pool A then insert back both SSD and try to configure SSD read-cache one in Pool A and another for Pool B
Hope this helps!
Regards
Subhajit
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