partitioning - Small SSD to Large SDD cloning, why isn't it allocating remainder of space?

I'm upgrading from a 120gb PNY SSD to a 500gb Samsung 850 SSD.

I cloned the 120gb SSD using EaseUS Todo Backup onto the larger SSD. But it seemed to copy the setup of my smaller drive. So on my larger SSD, I have 354gb that is unallocated.

How can I merge the primary partition and the unallocated?

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Update

When trying to extend the volume via disk management, I get the following message: