Hi all, I have an iMac late 2015 base model with 1TB HDD internal storage. Could someone tell me the read and write times of the drive and also I am considering of buying an external SSD(256 gb), how fast is an external SSD compared to the interal HDD?
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internal HDD read and write speeds
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Sequential Read/write speeds actuallly mean very little in day to day usage to be honest. As to your question it will depend on how full it is and the speed of the drive the 21.5 inch machines are 5400rpm and will give you somewhere between 80mb/s and 150mb/s read write sequential depending on lots of factors, and the 27 inch uses a 7200rpm drive that’s a little faster 100mb/s to 160mb/s sequential read/write. Even a usb 3 ssd external drive will give you 550mb/s read writes, making them a lot faster maybe 3x as fast however the small operations will be 10x as fast in some instances.
SSD’s are much faster in all operations due to very low seek times. Basically even with a USB 3 connected external ssd for your OS X and apps, you will be looking at 10 to 15 seconds boot time and almost instant app opening, of course any media etc stored on the HDD will be a little slower and it will help to keep any projects you are working on on the ssd as well but it will vastly speed up operation of the computer.
SSD’s are much faster in all operations due to very low seek times. Basically even with a USB 3 connected external ssd for your OS X and apps, you will be looking at 10 to 15 seconds boot time and almost instant app opening, of course any media etc stored on the HDD will be a little slower and it will help to keep any projects you are working on on the ssd as well but it will vastly speed up operation of the computer.
4K random read is a very good reference for "OS operation" performance (e.g. boot time). So, a SSD can be ~30x faster than a HDD regardless the connection type. PCIe (both ACHI, NVMe), SATA 2, SATA 3, USB 3.0, Thunderbolt.... doesn't really matter. Unless you use something like USB 2.0 which only provide little bit buffer over the 4K random read speed, the OS operating speed generally won't be limited by the connection type (USB 3.0 is definitely fine).
OP wrote:
"I am considering of buying an external SSD(256 gb), how fast is an external SSD compared to the interal HDD?"
If you set up an external USB3 SSD as your boot drive, the iMac will run much MUCH faster.
Probably 3-4x, if you care to measure the reads/writes using BlackMagic Speed Test.
You could either buy a "ready-to-use" SSD such as a Sandisk Extreme or Samsung t5, or...
... buy a "bare" SSD and a 2.5" USB3 enclosure and "build it yourself".
Be aware that if you buy something like the t5, it probably comes with some factory-installed proprietary software with a custom partition, special apps, etc.
These are likely to "get in the way" of a fresh Mac OS install.
Therefore, you'd need to REMOVE this stuff before setting up the drive for the first time.
This won't be a problem with the "bare drive" scenario.
You don't need a large SSD -- 500gb or even 250 can do the job.
Put your OS, apps and account on the SSD.
Leave "large libraries" behind on the internal drive -- things like movies (possibly large libraries of pics or music as well) -- they don't require speed to run, they can "live" just as well on the slow internal HDD.
I've been booting and running my 2012 Mac Mini this way for FIVE years now -- still runs like new.
"I am considering of buying an external SSD(256 gb), how fast is an external SSD compared to the interal HDD?"
If you set up an external USB3 SSD as your boot drive, the iMac will run much MUCH faster.
Probably 3-4x, if you care to measure the reads/writes using BlackMagic Speed Test.
You could either buy a "ready-to-use" SSD such as a Sandisk Extreme or Samsung t5, or...
... buy a "bare" SSD and a 2.5" USB3 enclosure and "build it yourself".
Be aware that if you buy something like the t5, it probably comes with some factory-installed proprietary software with a custom partition, special apps, etc.
These are likely to "get in the way" of a fresh Mac OS install.
Therefore, you'd need to REMOVE this stuff before setting up the drive for the first time.
This won't be a problem with the "bare drive" scenario.
You don't need a large SSD -- 500gb or even 250 can do the job.
Put your OS, apps and account on the SSD.
Leave "large libraries" behind on the internal drive -- things like movies (possibly large libraries of pics or music as well) -- they don't require speed to run, they can "live" just as well on the slow internal HDD.
I've been booting and running my 2012 Mac Mini this way for FIVE years now -- still runs like new.