Am thinking of replacing some old backup drives and leaning towards 2.5" because of their smaller size and the fact they do not need a power source.
Have there been any past indications that larger drives are better in reliability or other factors. Thanks for your advice/suggestions.
Don't make assumptions about power, the 2.5" Notebook drives and the 3.5" Desktop drives use the same SATA connections, just there will be cables with the larger and a socket inside a Notebook for the smaller. Check the labels on the drives for their power needs, could be 12V DC and 5V DC with varying current needs. There are USB drives that are pocketable that can live with the power provided by 1 USB port. There's also empty cases for the 2.5" that use a Y-cable to provide power and data and additional power [2 USB ports]. Most all of the larger portable 3.5" drives come in cases with their own power supply. There are adapters for Desktop cases to allow use of the 2.5" drive in a 3.5" bay, for example the SSD drives.
I use a mix inside my computer and have no problems at all with any of them. They all work together perfectly.
I generally shy away from 2.5 inch drives used internally unless SSDs as they are usually slower than 3.5 inch drives. They are probably slightly less reliable due to compact size.
However, if drives are for backup, the demand on drive will be low.
For external usb backup drives, 2.5 are far more convenient as they do not require their own external power source.
They all need a power source from somewhere.
As always there is a large chance factor in their failure.
For 3.5" drives you can buy higher reliability Enterprise drives which are also more expensive.
https://research.google.com/archive/disk_failures.pdf
3.5" drives use 12v and 2.5" use only 5v. USB supplies only 5v so 3.5" drives need separate power supply. 2.5" HDDs are generally slower than 3.5" counterparts even at same revs which doesn't impact on USB2 but on USB3 are a bit too slow to keep up with it at full speed. For intermittent use as backup it's not very important. 2.5" drives are also less susceptible to damage due to moving them and handling them while working and also made to be more resistant when subjected to higher Gs when not running, partly because of less inertia of smaller parts and partly because they are made to be subject to a lot of moving. Otherwise I haven't seen any difference in longevity if used properly and have a mix of drives used as external backup.
Quote