Primer on SSD and internal hard drives

Whoa, a topic with general consensus! That never happens here. OK,  Spinning disks are the past. SSDs are the future. No moving parts so less power, less heat, longer life, and absolutely no SQ difference. 

Spinning disks deserve a special place in history for helping put the modern world online. The original iPod used a postage stamp sized Toshiba 4GB spinning disk that was the pinnacle of technology 20 years ago. Today I keep my iPod full of 160 and 320K MP3s as a reminder of how fast technology moves. My current phone has 256GB of SSD full of uncompressed FLAC files.  

SSDs' are quicker, zero moving parts, and less power draw.  As time goes by, even the larger drives' prices continue to edge down.  Smaller than a SATA as well.

Considering swapping all drives over to SSD, but don't forget to max out your RAM.

Spouse just punched her desktop up and the change was immediately noticed.

Shop around like you do for your 'anything else'... ;)

To answer your specific questions:

So what is the difference between an internal hard drive and an SSD?  Is one preferable? ----- SSD can be external or internal, the comparison is a spinning disk or a SSD (explained above quite nicely). I prefer SSD for music, no possible noise at all. In theory SSD are more reliable, but you should have backups anyway.---

The manual suggests using either a 3,5" or 2,5" drive. Does size matter ? ( no pun intended) -----No. However 2.5 are even more expensive then 3.5, so unless there is a technical/space reason given by the manufactor (maybe you can fit two 2.5's and only one 3.5 for instance) than get the 3.5 SSD.----

Are there sonic differences in the different available brands? ----I've used many brands, I have not heard a difference, and there shouldn't be one with an SSD. I just go with the one that has the best warranty and specs for long life.----

The failure rate of a mechanical HDD is greater than that of an SSD. That would be number one concern as I don't believe the sound quality of an SSD vs mechanical is any different. And ALWAYS keep at least one complete back up of your digital music. Storage is inexpensive. I use Samsung SSDs for all storage in my DACs and music servers. $120 for a 2TB internal drive. $204/4TB, $360/8TB all on Amazon.

Enterprise grade spinning drives are ideal for backup.

Good hygene is to have a bakcup of your backup stored in a different location.

The downside of enterprise grade drive is that they can be noisy so I don't use them any computer that needs to be quiet.

SSDs are faster, dead silent and the preferred solution for pretty much any form of computing.

NMVE drives are faster than conventional SATA SSDs and ideally suite for storing the computer OS - provided the motherboard supports it. However, its worth noting that some motherboards have an M2 form factor drive slot but it may not be NMVE compatible.