Never used SSDs as scratch disk or even after effects extensively. Just a noob. what is scratch disk? why is it necessary ?
Can I use a 250 GB SSD partitioned to load OS and another partition as scratch disk? or is it necessary I need 2 SSDs.? what is minimum size SSD required for a beginner for using as scratch disk?
Ae can use a disk to cache rendered layers and frames. This is meant to speed Ae up; rather than re-rendering a layer or frame that Ae has already processed before, but that is not currently cached in RAM, it can go grab it from the disk.
This writes lots of little files -- one per layer, plus one for the comp, for each frame cached. Accessing this number of files can be slow with a mechanical hard drive, but it's pretty speedy with an SSD. That's why we normally recommend large, fast SSDs for Ae's disk cache.
If you're using only a single disk, partitioning the drive isn't helpful from a performance standpoint, but it would help you to allocate space more carefully.