Modern PCs tend to use one of two types of internal-storage devices:1 2
- Hard-disk drives (HDDs) store data on spinning disks coated in a magnetic recording medium, a technology dating back to the 1960s. HDDs are very cheap and available in enormously-high capacities, but relatively slow and mechanically quite fragile (they have lots of parts moving really really fast that have to be kept in extreme close proximity to one another without colliding).
- Solid-state drives (SSDs) store data in semiconductor memory, either flash EEPROM or (occasionally) battery-backed DRAM or SRAM. SSDs are faster than HDDs and far more mechanically robust due to their lack of fragile moving parts, but are also much more expensive per byte, not available in capacities nearly as large, and less reliable for long-term storage (as their memory cells tend to slowly leak electrons over time).
Both types of storage device wear out over time and eventually fail. However, it seems to be common knowledge that SSDs always, or almost always, fail catastrophically, all at once, with little or no advance warning; in contrast, HDD failure is usually gradual, with a long period of slowly-declining performance and increasingly-frequent occasional fsck/chkdsk-warranting read/write errors, providing plenty of time to back up the contents of the drive and swap it out for a fresh specimen.
While both HDDs and SSDs have obviously-catastrophic failure modes (such as a failure of the drive controller or interface), one would expect, for both types of drive, that most drive-wearing-out failures, at least, would occur gradually, in the manner classically associated with HDDS, with the drive gradually becoming slower, encountering increasingly-frequent read/write errors, and accumulating bad sectors (for HDDs) or memory cells (for SSDs) over time, until the degradation finally corrupts something vital to the operation of the drive.
Both types of drive deal with worn-out portions of medium in the same manner, by silently remapping data from dead sectors or cells to an internal store of spare sectors/cells set aside at the factory for this purpose;3 even once this stash of spares has been exhausted, and bad sectors or memory cells start becoming visible to the OS using the drive, the filesystem used on the drive still provides a second layer of protection by remapping from these bad portions of storage medium to unused, still-good storage.
Indeed, one might naively expect that HDDs would be somewhat more prone to sudden, catastrophic failure than SSDs, for a couple of reasons:
- HDDs possess catastrophic failure modes not present with SSDs (such as a failed or seized spindle motor or a head-disk collision).
- With HDDs, the lowest level of granularity is the sector (a chunk of drive 512 bytes in size, or 4 KiB on some newer drives), whereas SSDs work with individual memory cells (storing just one to four bits each), allowing much-finer-grained handling of good and bad sections of the storage medium and permitting more efficient use of what good space remains on the drive.
Yet, despite all this, HDDs usually fail gradually, while SSDs usually fail all at once. Why? Could this be related to how most SSDs start redistributing writes away from highly-used memory cells even before they start to fail, delaying the first few outright cell failures (but, even then, one would expect there to still be considerable advance warning of an impending drive failure, as individual cell failures are statistical rather than deterministic, making it impossible to eliminate the early-warning cell-failure tail)?
1: Obviously, a computer is not restricted to using just one or the other; as long as it has enough connectors to connect two or more internal drives, it is perfectly possible to install both an HDD and an SSD in the same computer alongside one another. This can be used to exploit the advantages of both types of storage device, with the smaller, faster SSD holding the operating system and disk-access-limited applications, and the bigger-but-slower HDD holding bulk data and non-disk-access-limited applications.
2: Some newer drives use both methods of data storage, combining a large, slow magnetic drive with a smaller built-in solid-state cache; these work on the same principles as the setup described in footnote 1, but generally appear to the operating system as a single drive, rather than two, and do not usually offer the user the option to specify whether files should be written to the magnetic storage or to the solid-state memory. Such drives are commonly known as fusion drives, not to be confused with the method of starship propulsion by the same name.
3: This is why modern HDDs and SSDs generally report as having no bad sectors or cells at all when new, despite the imperfections inherent in drive manufacturing, and why bad sectors/cells only start becoming visible to the OS late in the drive's lifetime, once the drive's internal stock of spares has been exhausted.