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Upgrading memory on an Optiplex 780 SFF

My Optiplex 780 SFF currently has 2 x 2Gb Dell (Samsung) Memory boards in white Slots DIMM1_1 and DIMM1_2

When I attempt to add a supposedly compatible SK hynix HMT451U6AFR8C-PB NO AA    1306 T   (4Gb) to either of the black slots  DIMM1_3 or _4 there is a rapid tapping noise in fits and starts and the computer doesn't boot FreeNAS. (which does boot OK with the original Dell memory).

Is it allowable to mix one 4Gb board with two 2Gb boards? If so does the board order in the slots matter?

Or is the hynix memory incompatible or likely faulty?

Any help greatly appreciated

CB

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