what is the usefulness of 3.5" hard drive?

It stems from the history of hard drives. Originally main stream hard drives were 5.25 inch, then reduced in size to 3.5 inch. 2.5 inch never really took hold in desktop computers but are mainly used in laptops. You don't get the same capacities in 2.5 either.

3½ inch drives were the original standard for use in the PC as that was the size of the standard Disk Bay set by the use 5¼ inch floppy disk and then the use of CD and DVD and eventually Blu-Ray media.

It's a good size as it tends to be more easily naturally cooled. (although I find that forced fan cooling of the drive bays in a desktop can help cool the whole enclosure, especially in Server applications)

The 2½ inch package was originally used to fit into the 3½ inch floppy drive bays and I can remember Laptops with a user swappable floppy drive/ HDD/CD /DVD Drive bay. The 2½ drives were used for space saving for use in Laptops but have always been liable to run hotter than the 3½ and thus have a shorter life, the size also allows the 3½ drive to run faster so improving data transfer and the larger platter diameter also allows greater data density and thus higher capacities.

Modern drives are using the 2½ inch package size as they are starting to default to digital not mechanical methods of storage, which does not have so much of a Heat issue.

The 2½ inch standard SSD package is of course now being superseded with new package types such as the M2 which is more like a Memory module than the SSD which still retains the old case format (2½ Inch)

There were 5.25 inch drives back in the day though, aka Quantum Bigfoot.

There have been many different disk sizes used over the years in Mainframes and also during the early days of the PC some larger sizes are still used in specialist sizes



I have seen a platter several feet in diameter so limits are vast

This is a recycled disk platter from the good old days

Ive seen that EEV BLog video, was very interesting.

You would have to check the actual recorded figures for individual drives from usage statistics from various manufacturers, but generally a hotter running electronic device will last a shorter time than a cooler running one, and 3½ inch drives generally run cooler than 2½ inch.

Also an external drive will in most cases have a less efficient cooling system that one mounted in a regular PC case or even a laptop so would be likely to be under most heat stress