How Do SSDs Work?

A solid state drive, or SSD, is a storage device that uses integrated circuit assemblies to store data persistently. Unlike traditional hard disk drives (HDDs) that rely on spinning magnetic platters and a mechanical read/write head, SSDs have no moving parts whatsoever. Instead, they store every bit of data using NAND flash memory, the same fundamental technology found in USB flash drives and smartphone storage, but engineered for far greater performance, capacity, and reliability.

The term "solid state" refers to the fact that data is stored in solid semiconductor material rather than on a moving mechanical medium. This design gives SSDs several inherent advantages: they are faster, quieter, more energy-efficient, and more resistant to physical shock than their mechanical counterparts. Since their mainstream introduction in the late 2000s, SSDs have fundamentally transformed computing performance, turning sluggish PCs into responsive machines and becoming the default storage choice for laptops, desktops, and data centers worldwide.

When you delete a file on an SSD, the operating system marks the file's logical sectors as available, but the SSD controller does not immediately know those NAND cells are free. Without intervention, the controller would treat deleted data as valid, forcing it to read, modify, and rewrite entire blocks when new data arrives. This phenomenon, known as write amplification, degrades both performance and endurance over time.

The TRIM command solves this problem. When Windows deletes a file, it sends a TRIM command to the SSD, informing the controller which logical block addresses (LBAs) are no longer in use. The controller can then mark those cells as invalid during its internal garbage collection process, consolidating valid data into clean blocks and erasing the freed blocks proactively during idle periods.

Garbage collection is the controller's internal housekeeping process. It runs in the background, moving valid data from partially-filled blocks into consolidated blocks, then erasing the now-empty blocks so they are ready for new writes. Efficient garbage collection is essential for maintaining consistent write performance, especially as the drive fills up. Most modern SSDs also reserve a percentage of their capacity as over-provisioned space that is invisible to the user, giving the controller additional room for garbage collection operations.

SSDs read and write data in fixed-size pages, typically 4 kilobytes (4K). When partitions are aligned to 4K boundaries, each operating system read or write maps cleanly to a single NAND page. When partitions are misaligned, a single logical operation can span two physical pages, forcing the controller to read or write twice. This misalignment can reduce SSD performance by 20 to 40 percent and increase write amplification, accelerating wear.

Partition misalignment most commonly occurs when cloning an older HDD that was partitioned under Windows XP or earlier, which used 63-sector alignment instead of the modern 2048-sector (1 MB) alignment. A naive sector-for-sector clone will replicate this misalignment to the new SSD, silently crippling its performance. Proper cloning software must detect and correct partition alignment during the cloning process.