speed of USB, SATA, SSD...etc.

[joonkimdds]

Hi everyone.
I have external HDD that probably uses 2.0 USB since it was bought like 8 yrs ago.

I am interested in getting a new external SSD/HDD for faster and bigger storage.
Here are my questions.

Q1) if speed of SSD read and write is only 500MB per sec what's the point of getting USB3.1 Gen 2, SATA3 (6GB per sec) or something that transfers much much faster than how fast SSD can run? Wouldn't it make more sense to use USB 3.0 since its speed is only 640MBps and very close to average SSD speed?

Q2) if I buy external SSD w/ USB 3.0, but the old computer uses USB 2.0, does it transfer at 2.0 speed? And does the cable that connects these two matter?

Q3) why is there even external HDD w/ USB3.0? USB 3.0 is really fast 500 MBps. I assume HDD is much slowwer (about 200mb/s) so I have no idea why HDD even need USB3.0.

 

[ShadyHamster]

Well currently there isn't much point in getting gen 2 for a standard sata based ssd.
Sata 3 is rated at 6Gbps (Gigabits), so around 600MB/s.

Plugging in a usb 3.0 drive into a 2.0 port will limit the transfer rate to 2.0 speeds. If you are plugging it into a 2.0 port then the cable doesn't matter, you need a 3.0 cable for a 3.0 port for the faster transfer speeds.

HDDs still benefit from usb 3.0 since their transfer speeds are around the 200MB/s speed, for 3.5" drives, using 2.0 limits the speed to around 48MB/s.

 

[k1114]

1. You get whatever connection you have/need. You want internal, you get sata, pcie or m.2. You want external, you get usb or tb. There are only a few situations that can even reach those max speeds since most are not sequential operations. That makes it even more irrelevant. Whatever ssd you see being 500MBps is most likely a limitation of the connection used. You will notice pcie, m.2, and tb are much higher because those are higher bandwidth than sata and usb. More to this answered in Q3.

2. Yes you get 2.0 speeds. The cable for 3.0 also has a different connection. https://www.mycablemart.com/images/usb_connectors_20_30.jpg

3. You mixed bits (lower case b) and Bytes (upper case B). Also you say usb 3.0 is 640MB in Q1 then say it's 5GB in Q3. Let's clarify. Sata 3 is 6Gbits not GBytes but you don't get 6Gbps (750MBps) because those are theoretical just like 5Gbps (640MBps) for usb 3.0. Because of overhead and other inefficiencies, you're looking at 600 and 500 MBps respectively which is probably why all the ssd you are seeing are around 500MBps. Usb 2.0 is about 35MBps real world and you should already experience this with your usb 2.0. This is too slow even for a hdd which usually are around 120MBps.

 

[cryoburner]

Transfers with USB 2.0 drives usually top out around ~30MB/s due to overhead, so even the performance of a regular hard disk will be significantly limited by them. The bandwidth offered by a 3.0 or higher connection should allow the drive to operate closer to its maximum performance capabilities.

If the computer only supports USB 2.0 though, you'll still only get 2.0 speeds. If it is a desktop computer with an open PCIe slot available, you could always add some USB 3.0 ports. Some USB 3.0 add-in cards can be found online for around $15 shipped, or potentially less if you are willing to buy direct from China through a site like ebay and wait a few weeks for shipping.

You would also need a USB 3.0 cable, since they include more wires to support the faster speeds, though I suspect that most external drives will likely include one.

 

[joonkimdds]

1. You get whatever connection you have/need. You want internal, you get sata, pcie or m.2. You want external, you get usb or tb. There are only a few situations that can even reach those max speeds since most are not sequential operations. That makes it even more irrelevant. Whatever ssd you see being 500MBps is most likely a limitation of the connection used. You will notice pcie, m.2, and tb are much higher because those are higher bandwidth than sata and usb. More to this answered in Q3.

2. Yes you get 2.0 speeds. The cable for 3.0 also has a different connection. https://www.mycablemart.com/images/usb_connectors_20_30.jpg

3. You mixed bits (lower case b) and Bytes (upper case B). Also you say usb 3.0 is 640MB in Q1 then say it's 5GB in Q3. Let's clarify. Sata 3 is 6Gbits not GBytes but you don't get 6Gbps (750MBps) because those are theoretical just like 5Gbps (640MBps) for usb 3.0. Because of overhead and other inefficiencies, you're looking at 600 and 500 MBps respectively which is probably why all the ssd you are seeing are around 500MBps. Usb 2.0 is about 35MBps real world and you should already experience this with your usb 2.0. This is too slow even for a hdd which usually are around 120MBps.

Oh...bits and Bytes are different? HAHA I didn't know that.

so, if we say SATA 3 = 6 Gbits = 6Gbps = 750MBps = only =600 MBps in reality = about the same speed as SSD.

USB 2 = 35MBps = still slower than HDD (120MBps)so HDD should still benefit from USB 3 although USB3 is faster than HDD.


And if M.2 is 10 times faster than SATA3, shouldn't SSD also be 10 times faster in order to keep up with trasnfer speed of M.2?

 

[k1114]

Ssd are currently capable of more than 600MBps but that limit in speed is caused by sata being at its limit. M.2 can run in sata mode and have sata speeds. But if you want the most out of m.2, you want to use nvme which uses pcie x4. When you start getting higher limits with nvme, you see the issue we saw when ssds were new many years ago, the ssd controller isn't good enough to max out the port being used plus higher density is also faster. The whole point of coming out with ports that have higher capabilities is so that when tech does advance, there is already room to use it.

 

[USAFRet]

Q2) if I buy external SSD w/ USB 3.0, but the old computer uses USB 2.0, does it transfer at 2.0 speed? And does the cable that connects these two matter?

Q3) why is there even external HDD w/ USB3.0? USB 3.0 is really fast 500 MBps. I assume HDD is much slowwer (about 200mb/s) so I have no idea why HDD even need USB3.0.

2. Yes, it would be at USB 2.0 speed.
End to end performance is dictated by the slowest device in the chain.


3. Because some new systems/motherboards are coming out with no USB 2.0 ports.
And it costs no more to have a USB 3.0 port on the HDD enclosure vs a USB 2.0 port. Better compatibility, better advertising...so why not?