Hard drives don't have an expected "lifespan" per se, other than a warranty and the MTBF listed in the datasheet. For modern drives the MTBF may be quoted as 2 million hours, but only have a 3 year warranty. They will fail when they fail.
A good model- and vendor-neutral rule of thumb is after 3 years of use, expect significantly increased chance of failure for every continuing year of operation. (I get this from experience, anecdote, and from information like
this, though see the next paragraph.)
Hard drive quality has improved in the last couple of years; we are seeing less failure rates in the field than we have in the past. (Others are too - see
this for example.)
Some drive models have more problems than others. Historically HGST (even under WD ownership) have been the most all-around reliable brand, and Seagate the least. Reliability depends heavily on model and technology shifts and drive handling more than the brand though. WD has models that fail more than Seagate's average, and Seagate has some models that are industry leaders in reliability. They have also had models that are the worst the industry has seen.
External drives are also the most likely to die, mostly from user handling that an internal drive does not experience.
Most people with heavy brand preferences have them based on a small number of drives and their past experiences. If they had good luck, they don't want to feel they could be wrong, so their brand choice is the only correct one. Others could have completely opposing experiences. I've posted Backblaze information in the links above, and while they primarily use desktop class drives, it isn't suitable information to draw a conclusion on drive lifetime in home use scenarios. Keep these points in mind if asking for drive recommendations. Look at the specific model and its failure rate if you can find it, keep the brand secondary.
A hard drive sitting idle and unpowered, undisturbed externally, should work and retain data for a long time - between a decade and a century or so. The most likely fault when powering up a well-used or long-sitting drive is the spindle seizing. Very old drives (which were simple) and new drives (with modern bearings and lubricants) should have no problems for a long time.
