EMZETT.Internal Hard Drives
In short: Mass storage permanently built into a computer’s case, connected directly to the motherboard via SATA or NVMe/PCIe.
In more detail: Since no external wiring or USB bridge is needed, internal drives tend to be connected faster than external ones — especially noticeable with NVMe SSDs, which run directly over PCIe.
In Depth
Internal drives are slowed down to different degrees, or not at all, depending on the connection type: SATA drives (both HDDs and older SSDs) are limited by the bandwidth of the SATA interface itself (a maximum of around 600 MB/s), regardless of how fast the internal storage electronics would theoretically be. NVMe SSDs completely bypass this limit by connecting directly via PCIe lanes — the same fast connection technology also used by graphics cards — and thereby reach speeds of several gigabytes per second, several times that of SATA.
Form factor also plays a role with internal drives: classic 2.5-inch or 3.5-inch enclosures are connected via SATA data and power cables and need corresponding space in the case, while M.2 drives are plugged directly and cablelessly into a slot on the motherboard — considerably more space-saving, which is why M.2 is now standard especially on laptops and compact desktop PCs. Important when buying: an M.2 slot doesn’t automatically support NVMe speed — some motherboards offer M.2 slots that only pass through the SATA protocol — checking the manual is worthwhile before buying an expensive NVMe SSD.
RAID with internal drives
Several internal drives can be combined into a shared logical array via RAID — e.g. to redundantly mirror data across two drives (fault tolerance) or to combine several drives for higher speed. This requires either a dedicated RAID controller or a software RAID solution in the operating system, and is considerably more common with internal drives than external ones, since RAID benefits from as direct, low-latency a connection as possible.
Why SSD instead of HDD as the system drive
For the drive the operating system itself is installed on, an SSD (whether SATA or NVMe) has become the standard over a classic HDD: the speed difference for random, small read accesses (exactly what happens during system boot and program launches) is many times greater for SSDs than for purely sequential reading of large files — HDDs nevertheless remain popular as an additional, cheaper internal drive for large amounts of data (archives, media libraries), where pure capacity per euro matters more than access speed.
See also: External Hard Drives, SATA, NVMe, HDDs, RAID