Best way to configure all drives for video/photo editing

I've done a tiny amount of video (Blackmagic Resolve) and a ton of Lightroom. One problem you have is that there are competing issues, notably I'd want redundant raid for images, but that's slower.

For video and Resolve, a huge issue is GPU(s); it heavily depends on them for rendering, especially with a lot of nodes in the color grades. It will likely be a limiting factor more than the disks, though they come right behind.

Let me answer Lightroom first as I'm more confident there. The Preview Cache, ACR cache, and your scratch and temp should be on either a single non-raid SSD, or a Raid-0 SSD. When there is a disk bottleneck, they are usually it (especially during interactive use as opposed to big exports). The preview cache has to be moved with a soft-link, as by default LR will only put it next to the catalog (which IMO should be on a raid-1 for safety). The actual image files do not matter a lot for LR, though for Photoshop if you are writing back large TIF's, having them on faster disks helps a lot. I kept all my images on slow spinning disks for years, and just changed to SSD, and frankly can't see a difference in that respect but most editing is in LR (which doesn't write big TIFF's normally). Putting the catalog file itself in faster disk does help quite a bit in some operations, though not develop (which is where most people feel LR is slow).

For most things single core speed is the biggest bottleneck for Lightroom, not disks.

My biggest concern in the above is that you have no apparent setup for any raid protection for your data. If that's OK with you, great. Just think about it.

With no raid at all, the fastest thing is the M.2 card, so I'd put your caches (Resolve, Photoshop ACR, Photoshop temp, LR cache) there as well as OS. Then just spread the rest of the load as widely as you can, observe which disks are busy and consider relocating things as you observe how they vary with your own workload. Static images (raw files not being actively changed) on the spinning disks, though you might move big video files to fast disk while you are editing them.

Sounds reasonable if you have space. Raw video is really large (and compressed works badly in Resolve and probably most editors).

I think the real answer will come from use -- monitor when you have wait times (e.g. deep queues) on a drive and if consistent, move stuff off. A lot of times the answer is really counter-intuitive, you never know what developers are doing under the covers. It's one thing that is so frustrating with Lightroom, it won't benefit from lots of cores; they do a lousy job of using parallelism. So the big honking X99 systems actually run slower than the faster Z170 ones.

Oh... I'd try with HyperThreading off as well. I did some benchmarks on Lightroom and found it was faster off than on. Now Resolve... might be the reverse, never tried it. Photoshop my guess (guess!) is that it's a wash. But it is definitely not always helpful.

Configure your scratch file/temp files folder to a separate SSD away from the OS drive.

Format your HD, if 95+% store for large video/photos to 32/64k size cluster to minimize fragmentation.

When you format a HD, you have the option to specify the CLUSTER SIZE, most people, not knowing what this is for, just take the default, which used to be (silly) 512 bytes, and I think it's now up to 1K with modern OS. The CLUSTER SIZE tells the OS to never break up a file below this size. This is the smallest size the OS will break up your files for storage. If your HD drive is mostly used for large files, it makes no sense to leave the OS to break up your large files into (relatively) small chunks. This is called fragmentation, the more your files are fragmented, the slowest is for the app to retrieve all the pieces, reason why some people go mental running defrag all the time.

There is a downside to increase your cluster size , is if you have lots of small files on the drive, say you format it for 64K cluster size, so even if a file takes a few bytes, the space it will take on the disc, is the minimum, 64kB. Todays though, most files tend to large, relatively anyway, compare when this was invented, call it legacy defaults.

Large drives may be different, but I just formatted a 4TB drive, and a 1TB drive, under Windows 10, and it set to 4096. Your point is a good one, just offering a data point. Also, if you are using raid it's useful to have your cluster size be evenly divisible into your stripe chunk size (or a multiple of).

As to system vs another drive for scratch/temp - only by some experimentation will you know if the added speed of the M.2 will offset the fact that the O/S drive is often busy doing other things. My educated guess is that for photo and video editing, the O/S will be mostly quiet, and you get more bang for your buck with its speed than you lose from lack of isolation. But experiment. The nice thing about caches and temp type areas is you can just delete and move them fairly easily to test.

Sure. Go to Computer Management, look for Storage, then Disk Management. In there you will see the drives on the left and their volumes to the right. Volumes have the drive letters in windows. Find the one you want to change, and right click on it, and "change drive letters and paths".

It is worth noting if you have already installed software using a drive changing its letter from that when you installed the software may break things.