How important is it to have a spare replacement disk (cold spare) ready to go at all times? My NAS runs Synology's Hybrid RAID in a 1-fail-tolerance system. All the disks are from the same batch, which I've read is kind of risky because they're likely to kind of fail together. But also I'd rather put off buying extra disks until closer to their death date when it'll be cheaper and I can potentially upgrade the whole set. Thoughts?

@s0 i have survived without a spare disk actually on hand for a good bit (hardware raid 5 with 4 disks). if a disk is acting up i will order a new one at the time and replace it

@s0 this is probably not a good strategy though. idk, i'm a hobbyist sysadmin. you should have backups so that you can eventually restore the system in the case of multiple-disk failures you're not prepared for