Some stuff like jpegs or mp3s are already compressed formats. Your not going to save much space compared to something like a plain text files.
Cyclic redundancy error is caused by CRC checking. CRC stands for cyclic redundancy checking, it's a way to check to make sure that files are correct. If you get a CRC error that means the file is corrupt, or not all their, or something else that would the cause the file to be messed up. It generally used in file transmission, like beind downloaded. Your IDE chipset and harddrive perform crc checks on data as it's moved back and forth on your IDE cable (although the instructions are checked).