In short: modern internal hard drives for PCs, laptops, NAS and recorders connect via SATA III. SAS is a server interface that works only with a SAS controller. IDE (PATA) is a legacy standard for old computers. External drives connect over USB, and for an HDD it should be USB 3.0 or newer.
SATA: the standard for everything at home
SATA (Serial ATA) has three generations with identical connectors and full backward compatibility:
| Version | Bandwidth | Practical limit |
|---|---|---|
| SATA I | 1.5 Gbit/s | ~150 MB/s |
| SATA II | 3 Gbit/s | ~300 MB/s |
| SATA III | 6 Gbit/s | ~600 MB/s |
A new SATA III drive works on an old SATA II board, just limited to its speed. For a hard drive you will barely notice: even the fastest modern HDDs only approach the SATA II limit, and SATA III has plenty of headroom. The version difference matters for SSDs, not HDDs.
A SATA drive uses two cables: a SATA data cable (thin, 7 pins) and a SATA power cable (wide, 15 pins). The data cable is usually not included with the drive, so if your PC has no spare, add one to your order.
Tip: for SATA drives the BIOS should be set to AHCI mode, not IDE, otherwise speed and drive features will be limited.
SAS: servers only
SAS (Serial Attached SCSI) is the server drive interface, with 12 Gbit/s bandwidth in the current generation, dual-port connections for redundancy and advanced diagnostics. Compatibility is key:
- A SATA drive can connect to a SAS controller.
- A SAS drive cannot connect to a regular motherboard SATA port: the connector is deliberately keyed so it does not fit, and the board will not see the drive.
That is why server series often come in two versions: SATA and SAS. Before buying an enterprise drive for a home PC or NAS, check on the product page that it is the SATA version. Server models are in the server hard drives section.
IDE (PATA): for old computers
IDE, also known as PATA or ATA, is a parallel interface with a wide 40-pin ribbon cable and a separate Molex power connector. It was used until the late 2000s. New IDE drives are hardly made any more. To revive such a computer, it is often easier to fit a SATA drive through an IDE-SATA adapter, if the BIOS allows it.
USB: the external drive interface
Inside an external drive is a regular SATA drive, with a bridge board converting SATA to USB. So the speed of an external drive is limited both by the HDD itself and by the USB version:
- USB 2.0 - about 30-40 MB/s in practice. That is a bottleneck for a hard drive: copying a large archive takes several times longer.
- USB 3.0 / USB 3.1 Gen 1 / USB 3.2 Gen 1 - different names for the same 5 Gbit/s standard. It is plenty for any HDD.
- USB-C is a connector shape, not a speed. A USB-C drive can run at 5 or 10 Gbit/s: check the specs.
Connect an external HDD to a USB 3.0 port (usually blue or marked SS), not to a USB 2.0 port on the front of an old case.
What to choose
- Home PC, laptop, NAS, recorder - SATA III.
- Server with a RAID controller - SAS or SATA, depending on the controller and drive bays.
- External drive - USB 3.0 or newer.
Drive sizes and case compatibility are covered in HDD form factor: 3.5" or 2.5".
