Unpowered SSDs slowly lose data

Original Article ↗ Hacker News Discussion ↗

Unpowered SSDs and other flash-based storage slowly lose stored charge, making them unreliable for long‑term “cold” backups compared to magnetic hard drives or tape. Commenters dig into how NAND physics, cell density (SLC/TLC/QLC), firmware behavior, temperature, and JEDEC retention specs affect real‑world data survival, and note that consumer drives often lack clear guarantees. Many conclude that flash should be treated as active or short‑to‑medium‑term storage, with periodic full‑disk reads or scrubs and redundant backups on different media essential to avoid silent data loss.

Practical implications for backups

  • Many commenters realized their “cold” SSDs (laptop pulls, shelf backups, unused game/arcade systems, etc.) are risky: several report SSDs that were fine when stored but dead or badly corrupted after a couple of years unpowered.
  • HDDs also fail, but tend to degrade mechanically or show bad sectors while still allowing partial recovery; SSD failures are more often sudden and total.
  • Several people store backups only on HDDs or ZFS/Btrfs NASes, and treat SSDs strictly as “in-use” storage. Others prefer paying cloud providers rather than managing media aging.

How and why SSD data fades

  • Explanations center on charge leakage from flash cells: programming and erase are probabilistic, and over time voltages drift.
  • Higher‑density modes (MLC/TLC/QLC) pack more levels into each cell, so thresholds are closer, retention is worse, and endurance lower; 3D NAND now uses charge‑trapping rather than classic floating gates, but the basic problem remains.
  • Retention strongly depends on program/erase cycles and temperature: more wear and higher temps shorten safe unpowered time.

Specs, standards, and uncertainty

  • Discussion of JEDEC standards (JESD218/219):
    • “Client” vs “Enterprise” drives have different power‑off retention requirements (≈1 year vs ≈3 months), but those specs apply at end of rated life (after TBW/DWPD endurance testing).
  • Consumer SSDs often don’t publish clear retention specs; commenters question the concrete numbers in the article and note manufacturers rarely talk about unpowered use.

Refreshing / “recharging” SSDs

  • Consensus: merely powering on is not enough; blocks must be read so the controller’s ECC can detect weak cells and rewrite/relocate data.
  • Firmware behavior is opaque and model‑dependent. Enterprise firmware often performs background refresh when powered and idle; consumer drives may do less.
  • Suggested user tactics: periodic full‑device reads (dd if=/dev/sdX of=/dev/null, pv, ZFS/Btrfs scrubs), or regular fsck/scrub schedules on always‑on systems. For truly cold drives, some recommend fully rewriting data periodically.

File systems, tools, and strategies

  • Strong support for filesystems with checksums and scrubs (ZFS, Btrfs, UBIFS/ubihealthd) to detect and auto‑repair bitrot when redundancy exists.
  • Others augment backups with hash databases, parity tools (par2), and multi‑media 3‑2‑1 strategies (multiple copies, different media, offsite).

Media choices for long‑term storage

  • For long‑term archives, commenters lean toward:
    • Spinning disks (with periodic spins and checks).
    • Tape (LTO) for serious archival, despite cost/complexity.
    • Industrial/SLC or NOR flash for niche, high‑retention needs.
  • Several stress that flash of all kinds (SSDs, USB sticks, SD cards, even console cartridges) should not be treated as “stone tablets” for decade‑scale cold storage.