this is what I see

why would there not be enough contiguous space?

will defraging the rg affect other luns?

is there enough contiguous space in the raid group ?

you must have deleted LUNs from this raid group. You will need to run defrag and only then you will get 1.8TB of contiguous space back. This is SATA so defrag will take a while to complete.

i don't remember what the default priority is but you can lower it to not impact performance.

        Defragmentation will not affect the content of other LUNs on the RG. It simply move the empty spaces from the RG to one end to make maximum possible contiguous space (moving all LUNs towards one end).

        As this operation is going on there will be a performance impact as the SP utilization will be more. It depends on the defragmentation priority you set on the RG. By default it will be set to medium . Other values available are Low  (will have lowest performance affect) and High (will have maximum performance affect).

        If you don't want to go for defragmentation and want to have a LUN of more capacity, you may go for creating a metaLUN

   Vipin V.K