Choose a period that matches the question
Quarterly, annual, and lifetime are separate Backblaze source periods. Keep a comparison inside one period, then use the exact model and capacity filters that match the drives you are considering. The source has different inclusion thresholds for each period, so a missing model result is better treated as no data than filled with a value from another scope.
Rows below the source threshold stay out of the normal result set. Turn on Show small cohorts only when you want to inspect one, and read its Thin cohort label as a warning that one failure can move the point rate sharply.
What is a cohort, and why does its size matter?
A cohort is the group of drives represented by a row for that model and source period. It is not every drive of that model ever sold. A model missing from the selected period might not meet the source's reporting threshold, so absence doesn't establish either good or bad reliability.
Drive days combine the number of drives with the time each was observed. One hundred drives watched for ten days contribute 1,000 drive days, as do ten drives watched for one hundred days. Those groups have the same exposure total but can still differ in age, workload, and operating conditions.
How is annualized failure rate calculated?
The explorer calculates annualized failure rate as failures / (drive days / 365) × 100. Drive days are the exposure, so two models with the same drive count can still have different evidence when their observation time differs.
The point rate is paired with an approximate 95% statistical interval based on the same failures and exposure. A wider interval means the observations leave more uncertainty about the underlying rate under the model. It is not a range of possible lifespans and not a 95% promise about a disk you buy.
Zero observed failures gives a point rate of 0.00%, but the interval still allows a nonzero rate. With little observation time, one failure can change the estimate sharply. Compare the failures, drive days, and interval together before treating a small difference between models as meaningful.
Can Backblaze failure rates predict a home NAS drive?
Backblaze reports an operational data-center cohort with its own workload, age mix, and observation period. This page does not inspect SMART history, firmware, workload, warranty terms, or your NAS topology, and its fleet result is not a personal failure forecast.
Filtering and selection use the bundled snapshot locally in this browser, with no network request during exploration. Share state puts only bounded filters and selected model IDs in the URL. Before acting on a result, check the drive's current SMART history and firmware, confirm the warranty or return path, and keep a tested backup. Continue with the storage tools or the ZFS pool capacity calculator when the next question is pool planning. The official Backblaze Drive Stats data page is the source for the published cohort data.