Please give the definition of a "scratch disk".!! Why use such terms like we are born knowing what they are???
Please don't use so many abrevations such as SDI, VTR, ect. without an easy place to find the definitions!!
The term "Scratch Disks" has different meanings, depending on the program in reference.
For Premiere Elements and Premiere Pro, they refer to special folders, where various "working files" are located. These working files are essential for Premiere to function. They consist of Audio & Video Previews (proxy files that Premiere uses to give the smoothest display of the Clips on the Timeline, in the programs' Program Monitor, and best Audio), Captured Files (Audio & Video files that are ingested into a Project, from miniDV tape, whether SD, or HDV), Media Cache Files (Conformed Audio files, CFA files and Waveform Display files, PEK files), DVD Encoding (the necessary Transcode files, that are used to author a DVD/BD).
The user can assign locations for these files, but the default, "Same As Project," has some real pluses, especially when a Project is complete, and needs to be deleted from the computer. One might alter the various locations, based on the I/O (Input/Output) sub-system, i.e. their disk drives.
In Adobe Photoshop, Scratch Disks are locations for the Photoshop working files, which will be .TMP files, that are created, when Photoshop is launched, and grow, as Images are Opened, and edited. Those TMP files are "virtual memory" files, and take over, when the system RAM has been used up. They are deleted, when Photoshop is properly shut down.
Several other Adobe programs also use Scratch Disks, and most are similar to what Photoshop needs.
In order for the program to work efficiently, it needs to borrow space on your hard drive to write temporary files. That area is called the scratch disk.