Why is sata faster than usb?

[Patrick_19]

Just wondering why is sata faster than usb?
Even using a normal HDD with sata 2 is faster than usb 2. I was just wondering why this is the case. I get write speeds of around 70 mb/s on a 10 year old HDD while write speeds on usb 2 are max 10 mb/s if im lucky. I know that these are the older versions but im still wondering why that was the case since both ways are just methods of transporting data right? And since a usb doesnt have any moving parts wouldnt that make it logically faster versus sata?

 

[Flyfisherman]

1. SATA l runs as 1.5 Gbit/s. This is the same as 150 MBytes/s
2. SATA ll runs at 3.0 Gbit/s or 300 MBytes/s
3. SATA III runs at 6.0Gbit/sec or 600Mbytes/s
4. USB 2.0 runs at 480 Mbit/s or only 60 MBytes/s

If this is confusing about Gbit/s and MBytes/s on SATA it is because it uses 10 bit port speed, meaning 8 data bits is for data transfer and two bits for error correction and therefore 1Gbit/s=100MB/sec transfer speed on SATA.

Now keep in mind, these are ALL theoretical numbers, especially the USB ones. In real life, USB runs at a bit less than half of its rated speed. You must understand the difference between 1 Mb and one MB. The first is a MegaBIT and the second is a MegaBYTE. A MegaBYTE is much larger than a MegaBIT. In Fact, it is eight times larger. So USB 2.0 is way, way slower than either version of SATA.

HDDs cannot even go as fast as SATA l. They transfer Data at about 80-100 MBytes/s. This is a very rough estimate as there are many different types of hard drives. Your limiting factor will most assuredly be the REAL USB 2.0 speed of about 20-30 MB/s.

Best regards from Sweden

 

[InvalidError]

SATA2 = 3Gbps, USB2 = 480Mbps wire speed / 250Mbps usable after protocol overhead.

With a good USB2 controller, you can get about 30MB/s out of an external HDD/SSD vs ~250MB/s with an SSD connected to SATA3.

 

[InvalidError]

HDDs cannot even go as fast as SATA l. They transfer Data at about 80-100 MBytes/s.

My 1TB WD Black disagrees: it does 140-200MB/s sequential.

 

[photonboy]

1) SATA and USB use completely different chips to transfer data.

2) On top of that, most USB hard drives have a USB-to-SATA chip in the enclosure that for USB2 is almost always running at 32MB/second.

So connecting a normal SATA HDD you're going through the SATA controller (which for SATA2/Sata3Gbps is limited to roughly 280MB/second sustained read).

Whereas a USB2 external drive goes from SATA (the drive) through a SATA-to-USB chip, into the computer then it goes through a USB controller.

3) The type of files being accessed matters as well, and the part of the drive being accessed. For hard drives the inner portion is about HALF the speed as the outer edge (due to rotational speed of the physical platter) and below 2MB file size the time per MB of transfer starts to increase a lot as the read/write head is wasting time jumping between file locations.

Summary:
So it's complicated. If you want to know more look up Wikipedia or other sources though you need some basic computer knowledge to understand some of this.

 

[photonboy]

HDDs cannot even go as fast as SATA l. They transfer Data at about 80-100 MBytes/s.

My 1TB WD Black disagrees: it does 140-200MB/s sequential.

Yes, and modern drives (HDD, SSD, and others) have a fast cache as well that can be well over 200MB/second. It's worth noting that cheaper SSD's tend to limit the cache to save money so once the cache fills up (i.e. SLC) and you start writing direct to the slower TLC memory the performance can plummet.

A Hybrid drive often contains 8GB of SSD so the software decides which files go in the faster SSD part and what goes onto the HDD portion. This uses the same basic algorithms from before when the files you wanted fast access to where put on the OUTER part of the HDD for faster access (such as boot files).

Also, hard drives if set to power down may take several seconds to spin up before you can access a folder. Even if the folder is on a quicker drive you tend to have to wait for ALL the drives to be running though it depends what you are doing.

 

[Flyfisherman]

HDDs cannot even go as fast as SATA l. They transfer Data at about 80-100 MBytes/s.

My 1TB WD Black disagrees: it does 140-200MB/s sequential.

Mine too. I have among several drives in my rig: one WD Black Caviar 2TB and also a WD VelociRaptor (10 000rpm) and a Samsung SSD 850 Pro 1TB (for programs) and an Intel SSD 520 180GB (for system and a few essential programs).
But the HDDs will never come anything nearby the speed speed of SATA II
As I said before about SATA I: This is a very rough estimate as there are many different types of hard drives.

Best regards from Sweden

 

[InvalidError]

But the HDDs will never come anything nearby the speed speed of SATA II

What is the absolute maximum USABLE speed on SATA2? All SATA3 SSDs are hitting a bottleneck at 510-520MB/s due to SATA command overhead eating an extra margin on top of the link-layer coding. That would put SATA2's maximum usable speed at about 250-260MB/s.

At 200MB/s is 80% of 250MB/s. I would not call that "nowhere near."