It is generally recommended to store the fractals created with Fred's Fractal Factory on an external hard drive. This is not strictly necessary. You can store the calculated fractals on any drive. 


However the data stored for a single fractal image can be enormous. A single fractal calculated to 10 levels of times2 zoom as described in the Overview can contain over a trillion pixels. Each pixel requires 2 bytes to store the Escape Counts, so the fractal can require over 2 trillion bytes of data. When this data is stored to disk, PNG compression is used, so you can generally reduce the storage requirements by a factor of 10. And for many fractals, compression buys you a factor of a hundred. Still, that means that a 10 levels deep fractal requires between 20 and 200 gigabytes of disk space. So using an external drive is a good strategy to manage the enormity of the data involved.


For basically idiosyncratic reasons, each frame of image data describes a frame that is 1000 pixels wide and 1000 pixels high. A single frame is stored as two files on disk. One file contains the low order bytes of the Escape Counts associated with the pixels. The other file contains the high order bytes of the Escape Counts associated with the pixels. Uncompressed, each of these files would contain 1 million bytes of data. With PNG compression, it turns out that many of these files contain less than 4000 bytes of data. This is true in regions of the fractal that are relatively featureless. It turns out that the vast majority of the files stored for a Mandelbrot or Julia fractal contain less than 4000 bytes after compression.


When a file is stored to disk, it will always occupy at least the minimum allocation unit associated with the file system being used. If the minimum allocation unit for the current file system is 100,000 bytes, then a file that only contains 12 bytes of data will still occupy 100,000 bytes of disk space when it is stored to disk. Since most of the files stored by Fred's Fractal Factory are less than 4000 bytes, we want to use a file system with a minimum allocation unit of 4k or less.


So..it is recommended that the minimum allocation unit be set to 4k bytes when the drive being used with Fred's Fractal Factory is formatted. Note that exFAT and FAT32 will typically default to minimum allocation units much larger than 4k for a one terabyte drive. For exFAT, the default minimum allocation unit is 128k bytes on a one terabyte drive. The NTFS file system generally defaults to a 4k minimum allocation unit on all sizes of drives. Fred works fine with drives formatted with larger minimum allocation units, but you cannot store as many fractals on those drives because space is wasted.