How safe are zipped files?

Are zipped files any less safe than non-zipped files? Are they vulnerable to corruption if a system crashes during unzipping, for example?

Basically is it OK to store valuable backup documents only as zipped files?

Yes, it’s even less risky than it used to be. In the past when we stored our zips on floppy disks and cheap CD-Rs that went bad in five years, it was more common to see zip files spontaneously self-destruct. Nowadays, there are many media options and easy access to server farms via Internet storage—not to mention the increased reliability of our own hard drives, and even common SSD technology. Stuff doesn’t corrupt-in-place nearly as much as it used to, and that really always was zip’s Achilles heel. Corruption during decompression should be theoretically impossible, unless the decompression application is behaving poorly, as that is a strictly read-only action.

In all, I would say the advantages of archival are going to outweigh the advantages of tens of thousands of files in a backup. Nice big, simple files, are more efficient, easier to move around, and protect special Mac attributes like labels and such that would die otherwise on FTP and Windows-based servers or disks.

Like all backup tech though, you should periodically spot-check. The backup is no good if it can’t be restored. So every once in a while just decompress a few zip files to make sure everything looks good.