Raid 5 harddrive failed but smart says its fine. Do I still replace?

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http://askubuntu.com – I have being running on three 2TB harddrives in software raid for a little under a year how under ubuntu server. After restarting my computer mdadm would not allow a boot because the raid was degraded. I booted in to ubuntu desktop usb trail mode thingy. After looking in disk until I managed to see that 2 drives were fine but one of them had seemed to have lost its partition tables, The smart report on it said it was perfect with nothing to report. I have added it back to the raid array which is going to take some time. But was wondering what made this happen? Can I stop it happening aga (HowTos)