SMARTReporter

This is one of those applications that you don’t know you need until it is too late.

From the Developer’s site:

SMARTReporter is an application that can warn you of ATA hard-drive failures before they actually happen! It does so by periodically polling the S.M.A.R.T.-status of your hard-drives. S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology) is a technology built into most modern hard-drives that acts as an “early warning system” for pending drive problems. Because SMARTReporter relies on the S.M.A.R.T. implementation of Mac OS X, it only supports ATA or S-ATA hard-drives, if you want S.M.A.R.T. support for your SCSI or FireWire hard-drive, send feedback to Apple.
SMARTReporter 2.0 can notify you of impending drive failures by sending e-mails, displaying a warning dialog or executing an application. The current status of your drives is always displayed through the customizable menu item.

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