CARJ DIVERTS FOR MAINT WHEN EICAS MESSAGE INDICATES RIGHT REVERSER UNLOCKED.

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IN CRUISE AT FL300; THE EICAS SYS ALERTED US TO A 'R REVERSER UNLOCKED' MASTER CAUTION MESSAGE. WE NOTED A SMALL VIBRATION -- BUFFET AT THIS TIME; BUT COULD NOT ASSOCIATE IT WITH ROUGH AIR OR A REVERSER UNLOCKED CONDITION. WE RAN THE QRH FOR R (L) REVERSER UNLOCKED MESSAGE WHICH INSTRUCTED US TO BRING THE R THRUST LEVER TO IDLE; TO PRESS THE EMER STOW BUTTON; AND THEN SLOWLY INCREASE PWR ON THE R ENG TO DETERMINE IF THE REVERSER WAS IN FACT UNLOCKED. IT ALSO INSTRUCTED US TO LAND AT THE NEAREST SUITABLE ARPT. WE DETERMINED THAT ZZZ WAS THE NEAREST SUITABLE ARPT; AND WHILE THE CAPT COMMUNICATED WITH ATC; I CONTACTED THE FLT ATTENDANT AND LET HER KNOW WE WERE DIVERTING AND TO PREPARE THE CABIN FOR ARR (IT WAS COMMUNICATED THAT BRACE POS FOR THE PAX WERE NOT NECESSARY). I THEN CONTACTED COMPANY DISPATCH AND MAINT VIA COMRDO AND INFORMED THEM OF THE SITUATION AND THAT WE WERE DIVERTING TO ZZZ. WE LANDED AT ZZZ WITHOUT INCIDENT; AND WE WERE MET BY CONTRACT MAINT; WHO SECURED THE REVERSER WITH PINS; AND THE R REVERSER WAS MEL'ED. WE CONTINUED THE FLT FROM ZZZ TO OUR DEST WITHOUT INCIDENT.