According the Seagate's current web site on the topic:
"MTBF is a statistical term relating to reliability as expressed in power on hours (p.o.h.) and is often a specification associated with hard drive mechanisms.
It was originally developed for the military and can be calculated several different ways, each yielding substantially different results. It is common to see MTBF ratings between 300,000 to 1,200,000 hours for hard disk drive mechanisms, which might lead one to conclude that the specification promises between 30 and 120 years of continuous operation. This is not the case! The specification is based on a large (statistically significant) number of drives running continuously at a test site, with data extrapolated according to various known statistical models to yield the results.
Based on the observed error rate over a few weeks or months, the MTBF is estimated and not representative of how long your individual drive, or any individual product, is likely to last. Nor is the MTBF a warranty - it is representative of the relative reliability of a family of products. A higher MTBF merely suggests a generally more reliable and robust family of mechanisms (depending upon the consistency of the statistical models used). Historically, the field MTBF, which includes all returns regardless of cause, is typically 50-60% of projected MTBF."
A MTBF of 1,000,000 hours is a mere 114 years...giving you an idea of the meaningless statistic of MTBF in terms of a single unit.
I have had a PC since the days of the IBM PC (over 30 years now) and in that time I have had three harddrive failures, including one less than ten years ago when my computer was fairly new. So my own experience of MTBF would be about 88000 hours, nowhere close to 114 years (admittedly harddrives today are probably better enduring than the ones of 10-20 years ago, but I did have one fail less than 10 years ago!
So Seagate's web site continues:
"Seagate's new standard is AFR. AFR is similar to MTBF and differs only in units. While MTBF is the probable average number of service hours between failures, AFR is the probable percent of failures per year, based on the manufacturer's total number of installed units of similar type. AFR is an estimate of the percentage of products that will fail in the field due to a supplier cause in one year. Seagate has transitioned from average measures to percentage measures....
Here is an example excerpt from a Product Manual, in this case for the Barracuda ES.2 Near-Line Serial ATA drive:
The product shall achieve an Annualized Failure Rate - AFR - of 0.73% (Mean Time Between Failures - MTBF - of 1.2 Million hrs) when operated in an environment that ensures the HDA case temperatures do not exceed 40°C. Operation at case temperatures outside the specifications in Section 2.9 may increase the product Annualized Failure Rate (decrease MTBF). AFR and MTBF are population statistics that are not relevant to individual units.
AFR and MTBF specifications are based on the following assumptions for business critical storage system environments:
8,760 power-on-hours per year.
250 average motor start/stop cycles per year.
Operations at nominal voltages.
Systems will provide adequate cooling to ensure the case temperatures do not exceed 40°C. Temperatures outside the specifications in Section 2.9 will increase the product AFR and decrease MTBF."
The approach you might take is merely to POWER DOWN a harddrive, unless you intend to write to or read from it. So creating a System Image to such a drive means powering it up and using it once a month, for example. Or to have data redundancy of files created each week, power up and copy files to your duplicate data drive once a week. Then, no matter the statistical failure rate, your backup/duplicate data storage endures the longest.
BTW, a fire-rated safe will NOT keep your stored harddrive from harm if the house burns down. When the fire is bad enough, the heat WILL destroy things inside. Houses in the Santa Rosa CA firestorm last October had chimneys destroyed when the heat caused concrete between bricks to fail. From a newpaper story:
"Santa Rosa Couple's Engagement Rings Survive Tubbs Fire Even Though 'Fireproof Safe Didn't Make It'
"The fireproof safe didn't make it, none of the paperwork made it, but the rings did," Sam Brinkerhoff said. "Just for a moment, in the middle of all that devastation, there was a little bit of hope"
A second newspaper story pointed out that only one in 40 fire-resistant safes kept its contents safe (but charred):
"Terry Andreasen, a local locksmith in the area, provided assistance to residents, opening charred and heat-warped gun safes after a wildfire that destroyed so many homes in Santa Rosa, on October 8, 2017. Andreasen opened over 40 safes during the wildfire season, but only one safe got his attention.
It was a Champion Safe from the 2005 Crown Series, and when Andreasen opened it, he found that a few items had been charred, but almost all of the safe’s contents were in-tact and largely unharmed."
Plastic pipe IN THE GROUND in Santa Rosa was damaged from heat, rendering them toxic and needing to be replaced before neighborhoods could be again inhabitable. Fire-resistant safes have their limits, and the only way to 'backup' your data with greater reliability is OFFSITE STORAGE, not in a safe in the same building.