I'm trying to keep my data well protected with backups, using multiple hard drives (basically swapping the backup disk out every week). For checking validity, I was originally planning on md5summing or perhaps only crc32 summing the content and verifying them periodically. I realize also that tools such as par2 exist that make error-correction files at the cost of extra disk space.
My question is if there is any reason to waste the processing time and disk space on error-correcting parity tools (which seem to be designed for usenet or optical storage) or if 2 or 3 copies of the files (one in use, two backups) plus simple hashing/checksumming for validity are enough?