Multiple errors when creating a picture in MiniMagick Ruby wrapper

Okay. This the background. I am trying to create different image patterns and merging them into one. I am using MiniMagick and it is a ruby wrapper for Imagemagick. I get the following message: unable to clone cache `/tmp/magick-1r313oY0′: No space left on device. What might be the reason? I am now simply trying to convert an color image into grayscale but this error is stopping me to proceed with my different image patterns design. Ruby is fine as I am able to run other programs easily. Thank you very much for your quick responses.