I’m putting a new system together and my 3 fastest drives are all different speeds. I would appreciate input on what should be the priority from fastest to slowest for the speed of the three separate dives for my sample libraries, my boot drive and my project-audio drive?
Lots of people will have opinions here…but really it won’t make as much difference as you might think…fastest disk for the OS
the rest - meh…any drive in the last 20 years can probably do as much audio as you will ever need…samples…well depends on the sampler…how often you load them…do they disk stream…
so many variables …wouldn’t lose any sleep worrying 
If it’s any guide - I’m using a mix of nvme m.2 and sata SSD - all that happens is you find other bottlenecks in they system…modern hard drives probably aren’t the problem.
Consider how you work. If you are using lots of Instrument Tracks with sample libraries make the samples disk faster. If on the other hand you are tracking audio on a lot of Tracks, like recording a band all playing together, then maybe make the project disk faster.
Or maybe you ignore the speeds altogether, 'cause it likely doesn’t matter, and decide what goes where based on size.
What drives do you have? I find for my OS an ssd drive is up in less than 30 seconds. Now I will probably only turn my pc on once in the day and leave it on. I do use SD3 drum vsti which loads into memory. Some kits I use are huge around 8gig and were always my longest waiting time as I needed to change and compare kits. I use m.2 for my samples now and loading is a couple of seconds. So in my case samples are the priority.
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I’ve been replacing my hard drives with SSDs for awhile now and the SSDs keep on getting faster as well and now the original ones I bought are nowhere near the pcie 4.0drives.
My storage drives are dinosaurs, though.