Grown bad blocks are generally caused by one of three issues, mechanical vibration/shock, internal particulates, or mechanical failure of either the spindle or actuator motors.
Mechanical failure does not produce isolated bad blocks, it produces continual bad block growth. If your OneTouch isn't continuing to grow bad blocks, odds are good that this is not your problem.
On the otherhand, isolated occurences of bad blocks (or in your case, sectors) are due either to mechanical vibration/shock or particulate. Generally, these are not an indication of any defect with the drive and are not reasons to request a replacement.
Transient events like mechanical shock can produce single instance of bad blocks for obvious reasons. You bang the drive while it's reading or writing, you can effect it's ability to perform that operation, much the same as hitting a phonograph turntable gets the record to skip. Clearly, errors resulting from something like this isn't a problem with the drive, it's a problem with the way the drive is being used. A replacement would have the same problem.
Even if you're not moving the drive around, there's still particulate. The disk drive is built in a class 100 clean room. This environment is nearly dust and particulate free. As a result, the interior of your HDD is very, very clean. But it isn't perfectly clean. Some environmental particulate is captured inside the HDD during assembly. And as the drive operates, it generates more (albeit at a very low rate. But there are moving parts inside the HDD. Moving parts have friction, and where there's friction, there's wear, and wear makes particles). Mostly, this stuff ends up in the "recirc filter", a filter designed to capture particulate as air circulates inside the drive. But the filter, like the clean room where the drive was built, is far from perfect. Some stuff always gets by. And, when you move the drive around, jostling it about in the process, you knock loose some of the particulate that was safely sequestered and that stuff blows around inside the HDD again the next time you spin the drive up.
Recording heads fly at heights that are fractions of the size of a dust particle. When they bump in to particulate, they tend to ride over and then hammer that particle into the surface of the disk (at 7200 or 10K or 15K RPM, the head is moving pretty fast relative to the surface of the disk. For 15K, nearly 90MPH). The particle is small, but the size of the magnetic bits on the disk are small, too. Where the particle hits, it damages the disk surface and the bits recorded there are usually rendered unreadable. The good news is, the drive has sophisticated error correction hardware that usually allows the lost data to be automatically recovered. In performing that recovery operation, the hardware assesses the severity of the error. When the amount of error "damage" exceeds a given "threshold" (defined by the HDD designers and built in to the Firmware), the drive will "replace" the block with a spare, moving the logical location of that block to a different physical location on the media, in order to preserve your data.
Portable drives (OneTouch is Maxtor's version, but this applies equally to all) are particularly suceptible to both mechanical and particulate induced errors because they're portable. They get moved around more than a typical desktop drive, and they tend to take some bumps in the process. These kinds of bad blocks, like those from mechanical vibration or shock, are pretty much par for the course. All drives eventually grow some bad blocks, portable or not, it's the nature of the beast. If you're concerned, check the drive again in a week. If it's showing more replaced blocks, you may have a real problem. If you're not still growing new "defects", replacing your drive is unecessary.