The shortest answer, is, No storage device, whether printout, drive or DVD is immune from the natural elements, that is, from eventual degradation, loss, or destruction. It is better to calculate the probability of loss versus the value of the data being stored. For example, if a few files from a primary computer are temporarily copied to transport them to another computer, the probability that the files needed will arrive safely to the office is very high, aside from losing the device in the car seat on the way there. And its a copy–so no permanent loss if lost in transit.
Nonetheless, with respect to the plethora of flash drives on the market today, some are far more reliable than others. The greatest danger lies in the sheer volume that such a drive can hold, and the fact that any flash drive may suddenly stop working without warning resulting in total loss of the stored data. The better manufacturers do extensive tests on these devices, so the chance of failure may be very low, but never zero. Therefore, the best guarantee against loss is duplicate drives, whether flash drives or ordinary drives. Simply said, never rely on a single drive to store data for long-term. Use two or more drives to store the same data and then store the drives themselves in different locations to insure against fire or theft. In choosing physical storage location however, consider that the usefulness of archived data ages with time, so backups for current operations must be continually updated. Thus, the drives must be reasonably accessible for periodic update.
Probability of failure: Suppose a newly purchased 128GB drive is used to backup all your computer documents, and that the probability the drive would suddenly fail this year is .01%. If an identical second drive is purchased, and the same data duplicated to it, its probability of failure would also be .01%. But the likelihood that BOTH drives would fail at the same time — very unlikely (yet not impossible, especially if both drives purchased at the same time from same lot of manufacturing). The likelihood BOTH drives stored in the same location could be destroyed, lost, or stolen is much more likely, but this isn’t a function of the drive reliability.
In any case, use of flash drives for long term storage of critical data is generally discouraged because the nature of such drives is that if PHYSICALLY damaged, it is essentially impossible to retrieve the data. In contrast, if a hard drive mechanism fails, it usually can be physically repaired and data fully recovered. The size of hard drives is also such that they are generally not carried around in one’s pocket. For practical reasons, a drive sitting safely on a desk is much more secure than a flash drive subject to all sorts of elements from baby slobber to melting in one’s car.
To read more, see this CNET.com article.