If I recall correctly MTBF figures apply at a family/model level, not at a device level. Therefore a MTBF of 2.5M hours means that they expect one failure for every 2.5M hours of run time across the all the drives they've ever made of that specific type/generation. Typically MTBFs are calculated based on component design parameters and are then tuned with failure statistics, though that is somewhat sketchy as once the warranty lapses reports of failures typically do not make it back to the manufacturer - but outside of warranty is when you can expect the most failures!

Anyway, in practical terms this means that if you have 1,000,000 of these drives in operation around the world (just to keep the numbers simple) you can expect one drive failure every 2.5 hours.....because every 2.5 hours you will have achieved 2.5M hours of run time. 

You cannot apply fleet-level stats at a device level....no drive will last an average of ~300 years...  ;-)