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Saw that as well, but there is a module that plugs into them in some capacity and it's not entirely clear what the module is or contains. They could "just" be connectors for that module and serve no other functional or expansion purpose.
 
looks like there's room for a storage module, there's even screws on the top case to accomodate a module.

the power connector, 2xSATA, and some variant of a SAS multilane connector (looks lik USB) but more like a 1/2 of a SFF-8643 connector.

There's also the lock switch and the compartment behind it on the other side of the PCB. Not sure what is inside that section and why it needs to be covered.

Plenty of PCI slots available to add your own raid card and basically any SAS enclsoure without having to use thunderbolt.
 
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