convert: subimage specification returns no images

[magick]

> convert -depth 8 -size 2048x2048 -delay 6.5 -scale 50% gray:abc.raw[750-999] abc.gif

This tells ImageMagick to start reading the first gray image at file offset 3,145,728,000. It then reads 250 images, each 2048x2048 rows / columns. This implies your image is about 4GB. Is this what you expect? If not, let us know how you expect the command to behave and we'll investigate.