MTBF is usually the Mean Time Between Failures, not Mean Time Before Failure, so what is this 1.6 million hours? The Mean Time Before Failure, might appear to imply that this is the average lifetime of a drive before it fails. A figure of 1.6m hours would put that at a little over 182 hours. In fact that's a nonsense - physical hard drives have expected lifetimes far lower than MTBF (although generally they become obsolete before they wear out).
What MTBF is a statistical number of total operating hours for a given item. For instance, if you have 1,000 of these drives and operate them for a year (about 10 million operation hours) then an MTB Fof 1.6m hours, using the normal definition, would mean that you might expect an average of 6 failures a year. However, that only applies during the rated operational life of the disk - if they are only engineered to last for about 10 years, there will be a rapid increase in failure rate after that period (and, for some devices you will get early failures too - hence the need for burn-in on some critical systems).
The subject of MTBFs is widely misrepresented as being an indicator of the expected operational lifetime of an item. It isn't. Too often manufacturers are less than precise over the meaning of the terms that they use and leaving out the critical ones.