Should I buy a new Mac?

No it was a hardware issue. The hard disk finally failed last weekend. Lucky for them (I was asking for my sister in law - I am a PC man) a new used computer was already in the mail. A dual core with a SATA drive. They had backed up their HD as I insisted.

 

Though I backup daily and don't worry, the S.M.A.R.T. monitoring attributes of ATA & SATA drives can be read periodically and compared, using a popular program on Sourceforge.

S.M.A.R.T. Monitoring Tools

They will run on both Windows and MacOSX (though best for Linux). Google and others don't think they're very useful. Although about 200 attributes are defined, a drive manufacturer will implement less than a dozen. From this, they determine by magic formula whether your disk is 'OK' or not.

My feeling is that S.M.A.R.T. is only useful in detecting 'diseased' drives: manufacturing defects, physical trauma, &c. It is less useful for drives that die of old age. Someone I was communicating with on the Apple Boards recommends replacing 2 1/2-inch hard drives every 5 years.

In any case, you can have it send a polite e-mail message when the value of a particular attribute has changed from its last reading. In the case of a sick drive, you can adjust it to poll the values every 30 minutes or so, though I don't know why one would. Well, you might want to order a new one before bad clusters start appearing in your backups, I suppose; and it could diagnose a problem definitively, as suggested above.