Umm... you're off a bit on the 2.5" drives. The 2.5" external drives are simply SATA laptop hard drives.
Nothing more, nothing less.
Exactly. Not designed to last long. Most external drives are built to be cheap. Not to be long term storage devices. Good to shuffle files around, not for long term backups. There are external adapters that let you plug in the disks of your choice. IMHO far better choice than pre-assembled ones where you often don't know what disk is inside, for backups.
Hard drive technology is pretty universal when it comes to the platters and their magnetic retention.
But construction is not. There are different quality levels. The choice of materials and parts, production line specs and quality controls, etc. etc. After all, are the low end Canon DSLRs and lenses built with the same materials and specs of the high end ones? After all the technology is pretty universal when it comes to CMOS sensors, shutters and lenses... isn't it?
There are drives designed for enterprise level use and durability but in this case we aren't talking about MTBF, we are simply talking about magnetic retention.
But MTBF matters too. Sure, you can use multiple copies to reduce the risk - that's why I always alternate new sets to ensure they don't come near the MTBF together - but disks are more complex devices and thereby with more points of failure.
The quality of the controller and drive electronics/firmware has impact on the "correctness" of data written and its lifespan, as well the quality of the magnetic layer used for recording. Why high-end disks usually come in lower capacities (and longer warranties) than the lower-end ones? Because until they can warrant the same quality, they don't risk to sell you a disk that may fail too soon because the technology is not mature enough.
I spent time for years with a friend that owned a data recovery company
I spend part of my working time with Very Large Database applications running on SANs (Storage Area Networks) made of hundreds of disks (and lately, a lot of flash ones as well). Also the company I work for produces also military spec hardware, which needs to work in far worse conditions. Guess I learnt a couple of things too
Anyway, companies like Facebook, Amazon, Google and the like are now faced with the need to store huge archives for decades (of course, unless they go bankrupt earlier) - because users in they early 20s or 30s expect them to store their "memories" for that long - and are experimenting with different technologies to keep data available at least near-line (as users are less likely to access often older contents) cheaply. And Facebook is now experimenting with blue-ray 100GB disks. If you store just the platters, and not whole disks, it's far cheaper.
Some companies (Sony and Panasonic) are working on "optical disks cartridges" able to store 1-1.5 TB each with RAID capabilities Currently Panasonic already offers the ADA cartridges, but they are only 50GB in size, and are somewhat a proprietary format. With the proper recording layer (one which won't decay if a few years, or decades) optical cartridges could become am interesting long-term storage for static data.