Hello! Whenever I open disk utility, I need to choose between 'calculate image checksum (CRC-32)' and ' calculate image checksum (MD-5)' before I can open disk utility. I always click on either one of them because otherwise disk utility won't open. I am wondering what it means. I called the Apple support about it, but the girl on the phone didn't see it on her computer and couldn't find any info about it. Maybe one of you could help me out? Thanks!