When recording fails and the interface reports no hard drive, work through the following.

1. Drive type: use surveillance-grade drives rated for 24/7 continuous multi-stream writing. Desktop drives frequently drop out or fail under that workload.
2. Power and interface: make sure both the power lead and the SATA cable are fully seated, and that the recorder PSU can carry all installed drives — a real constraint on multi-bay models.
3. Initialised? A new drive must be initialised and formatted in the recorder, otherwise it shows as unformatted and will not record.
4. Bay and capacity: a drive moved to another bay may need re-initialising, and a capacity above the recorder's supported maximum will not be recognised.
5. SMART status: review SMART data. Growing reallocated-sector counts or bad-block warnings mean replace the drive and migrate the data promptly.

Check drive health and the number of days actually retained once a quarter, so the retention requirement is genuinely met.