Solved: HP Elitbook 840 g1 - SSD M.2 slot - HP Support Community - 5576316
HP Elitebook 840 G1 Hard Drive Replacement - iFixit Repair Guide
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Amazon.com: Replacement Hard Drive HDD SSD Caddy Frame Bracket for HP EliteBook 820 840 845 850 855 725 740 745 750 755 G3 G4 ( Not fit G1 G2 Model or Any Other Model ) : Electronics