Do internal SSDs challenge the conventional wisdom of storing digital assets on external "scratch" disks? I've heard that all project assets should be stored on a separate "scratch" disk for the sake of increased performance. I'm wondering if that conventional wisdom holds even when your "scratch" disk is HDD and your internal disk is SSD. (I'm aware that the cost/capacity ratio is lower for HDDs. My question concerns performance issues, not capacity.)

Here's how my question originated: I am experiencing lag using my mid-2012 MacBook Air (i5, 8GB RAM) to edit Final Cut Pro X. I'm wondering if re-arranging my assets would stand a change of improving performance. Here's where things are right now:


The application (the *.app file for Final Cut Pro X) is on my internal SSD.
The library (the *.fcplibrary file) is on my thunderbolt-attached HDD.
The digital assets (stills and video) are on the same external HDD.


My library file merely references the digital assets; I am not copying them into the resource fork. For that reason, my library file is small enough that I could store it on my local drive. The new arrangement would be:


The application (the *.app file for Final Cut Pro X) is on my internal SSD.
The library (the *.fcplibrary file) is on my internal SSD.
The digital assets (stills and video) are on the same external HDD.


So, would I be breaking some law of the universe with this configuration? And where did the conventional wisdom on "scratch" disks come from, anyway? Does it still apply to SSDs? (Again, in terms of performance only please, not capacity.)
Scratch disks should never be external.  I'm not sure where you heard that.  External drives can be used for long term asset storage, but it is best to move assets local to work on them.

The point of scratch disks is to be the highest speed work area you have available to work from.  Scratch disks are used for data that needs to be moved in and out of memory quickly, so internal SSDs are the best bet.

The one thing you don't want to do is you don't want assets and scratch disks on the same magnetic drive because of seek time.  This isn't a concern with SSDs though, so you can generally have them on the same drive.  If you are working with very large files, then there may be a bandwidth advantage to having them on different SSDs, but in order of importance you want memory to be the fastest possible, then scratch disk, then working asset storage, then long term asset storage.

The point is to keep data supplied to the CPU and GPU as much as possible.