Every hard disk (and even a solid state disk) is bound to fail sooner or later. The most frequent sign of trouble is when all applications show the spinning beach ball for multiple minutes and the rate at which this happens increases rapidly over the course of a few hours or days.
Another alarming sign is when your Mac suddenly fails to boot.
Lastly, if you see entries like the following in
/var/log/system.log and the disk device in question is a hard disk, that’s another strong sign of trouble:
Dec 30 15:42:25 MacBook kernel: disk0s2: I/O error.
The first thing to do is to boot from your Mac OS X Install DVD and run “Disk Utility” “Repair Disk” on the hard drive in question. Also check for the SMART status (bottom right of Disk Utility’s window).
However, even the status is “Verified”, this does not guarantee all is well, unfortunately.
If after the repair you still have trouble, get Smart Utility from Volitans Software and see if it shows you any attributes in red type, as shown below.
Then click “Show All” in the “Attributes” section and again check for any red type. If it looks as follows, backup all data you can and get an exchange for your drive.