I had a hot spare in the form of a backup drive. It was a 12 TB external WD that I'd already burned in and had as a backup target for the NAS. Then when one of the drives in the NAS failed, I broke the HDD out of the enclosure and used it to replace the broken drive. It hadn't been in use for many months and I'd rather sacrifice some backups rather than the array. I also technically had offsite backups for it that I could restore in an emergency.