If there were any .PAR or .PAR2 files included with the RAR archive then the missing part can be recreated using SmartPar or QuickPar respectively but without these PARity files I believe you are stuck. click here for more info on Rar and Par files.
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Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.