For example: I recently bought 2 disks. First unit, 9 months ago, a 1 TB last generation Seagate HDD... within around 100 hours of running time, after a big file copy operation the disk unexpectedly collected 300 reallocated sectors. Now, each time I power it on, it loses (reallocates) ~ 30 sectors. Today it has more than 3000 reallocated sectors and is still running only for experimental and curiosity purpose
Second unit, a 500 GB Western Blue HDD; after only 25 running hours I had more than 500 reallocated sectors => replaced under warranty by the reseller.
All aging disks tend to lose (reallocate) sectors sooner or later. On a fresh new unit, reallocations are always caused by a malfunction and/or a hardware failure. Install and setup the package named "smartmontools". It will monitor your disk and send you an alert when something goes wrong. No alarm does not mean that the disk is actually healthy but on the contrary, alarms are always predicting a more or less imminent total failure.