What is the difference between fusion drive and flash storage on iMacs and which one is better

A simple explanation: A fusion drive consists of two parts: one is a smallish SSD and the other a large spinning disk drive and they are combined and work as one unit by the OS. Flash Storage is an SSD (in various sizes) which is slowly taking the place of your regular spinning drive because it is much faster.

A Fusion drive is a "fusion"of a smaller capacity Solid State flash memory drive (also known as an SSD) with a conventional mechanical, spinning platters hard drive.

The other option is simply a much larger storage solid state flash memory drive (SSD) that elimates a mechanical spinning platters hard drive for large mass data storage.

This is the same type of storage that is used in Apple's mobile iDevices.

The Pros and cons.

The Fusion drive is cheaper.

The Fusion drive uses a larger capacitiy standard/conventional mechanical spinning hard drive for bulk data storage and the flash memory storage drive part (SSD) of the drive is a much smaller storage amount to install the OS, applications and, maybe, have a little extra room for data storage on the flash drive (SSD) portion of the fusion drive.

The Pros are that the OS start up and application launching are much quicker.

Fusion drives combine new storage technology with older known and, in some ways, more reliable and predictable storage technology.

The cons are, depending on the size of the SSD portion of the drive ( which is a variable in some model Macs, the Solid State drive storage sizes are as small as 24 GBs in size, to either 64 GBs,,128 GBs or 256 GBs in size), once the SSD drive is full, the rest/bulk of your data is stored to the slower mechanical hard drive portion of the Fusion Drive.

This means that data read/write access speeds are much slower than that of the solid state drive (SSD) portion.

To make matters worse, on cheaper model Macs, Apple couples smaller sized SSDs to slower 5400 RPM laptop computer standard mechanical hard drives rather than use the desktop computer standard 7200 RPM mechanical hard drives.

The total solid state flash drive only (SSD only) is much more expensive ( prices are always coming down) because the storage sizes are much larger starting at 256 GBs, 512 GBs, 1 and 2 TBs in size.

These types of solid state drives offer full time quick speeds, performance and access to data because EVERYTHING is stored on a very fast, very quick, large mass storage SSD!

The only real con to SSD is that it is still, relatively, new and young data storage technology, compared with the older and more conventional and arguable reliability of mechanical hard drives that have been with us for nearly 30 or so, and have some very real unknowns like how long will your data stay recorded onto the flash memory and stay intact.

SSDs are much faster than the Fusion Drive hybrid!

I hope I covered this and explained these two different storage mediums well enough and simplified enough so you understand the differences.

Good Luck!