Does it trash the disk if I interrupt the secure erase option of Disk Utility?
There's nothing on the disk to hide, so I don't want to waste time, but if I triggered it accidentally — blame the cat :-) — will that render the disk unusable?
You can interrupt a secure erase mid-process. Will it render the disk unusable?
Yes and no.
Yes in that anything written will likely be severely corrupted. Any pre-existing data is toast. No in that your drive will not be physically damaged. You can re-partition and reformat to work with Apple computers, Linux/Unix, and PCs.