Transfer rate/speed difference between external SSD and SSD in enclosure

Nov 23, 2018
Hi, I have a question and I guess a bit of a problem with transfer rates/speeds with some of my external storage devices.

The question is in regards to the Transcend ESD400 portable SSD drive with the capacity of 128GB. I bought this drive about a year and a half ago as it was the cheapest external SSD available and it has since served me well. I plug it into my PC via USB 3.0 port and when copying data to it (usually media files ranging from 2-3GB up to 70GB) i get transfer speeds of around 140-160 MB/s. I don't know if that's good or if it should be more but it starts at that rate and stays at it all the way through.

A few months ago I decided to get another external storage device and this time I decided to try the USB enclosure route- I bought a Patriot Burst 240GB Sata3 SSD and with it I got the Icy Box IB-223U3a-B enclosure ( Again, no real idea if it's any good but it seemed affordable and it has UASP support which I read was important. I got another one to use with Kingston UV400 120GB drive that I had around.

Anyway, transfering the same types of files to the new external drives resulted in faster transfer speeds at the start (with Patriot Burst drive) but it would eventually drop off significantly (from 180-200 MB/s to 50-60 Mb/s) resulting in lower average transfer speed and taking more time than with the Transcend.

I did some testing with Crystaldiskmark and the Patriot Burst in enclosure scores much better results there than the Transcend. My question is why does one have that drop in transfer speed and the other one does not? And is there any way to "avoid" that drop (maybe a better enclosure) or is it just the way it is? Is there any way to know when looking at the external SSD drives whether it will have a constant transfer speed?

Hopefully someone will be able to help with some information (and excuse my lack of knowledge on the topic). Thanks!


Jul 8, 2015
As to the difference between the initial transfer speed (with Icybox being faster) that might be a result of one supporting UASP and the other not. UASP makes SSDs faster over USB than traditional USB 3.0.

As to the drop off in speed during large transfers, that's almost certainly the SSD itself at the root of the problem. SSDs will typically have a RAM buffer used to hold writes until it's convenient to actually write them to the NAND, especially if it's needing to move around some data or handle some T.R.I.M. operations to prepare some blocks for the data.

SSDs have a handicap of only being able to write individual ones but not zeros, so to write a zero requires erasing an entire block and rewriting all the contents along with the changes. This requires moving around a lot of data during idle times and for this reason, SSDs employ both a RAM write buffer as well as some NAND zones for temporary write storage. When you hit the limit of both these buffers, you're suddenly locked to actual speed it can write to and move around data on the NANDs directly (e.g. 50-60Mb/s). I've actually seen this effect drop an SSD down to 10-15Mb/s in some cases with very large data transfers (such as cloning one SSD to another that wasn't fully reset first).

Most likely before, when you were using the Transcend one, it wasn't quite fast enough to ever fill the buffers, so you didn't ever see the drop-off. Also, you probably had less data on the SSD so it wasn't having to move around as much data to perform write operations. Now that it's getting a bit more full, and you're using a faster adapter, you're hitting the limits.

As to Crystal Disk showing one as faster, I'd bet if you switched the order of what enclosure you test first you'll get the opposite result. The SSD was probably still moving around data from the first test (which it doesn't know isn't a real write operation) when you started the second test.

Also, you may want to check to be sure that T.R.I.M. is supported and turned on for the drive (in either enclosure). If T.R.I.M. isn't enabled the SSD will get some serious lag over time.

SSDs actually can underperform a good hard drive for single large file transfers. But, they're 100x faster for all the tiny read/write operations done by an OS. That's why most people use an SSD for OS and program files, and a HDD for all the big file storage. SSDs don't make a lot of sense yet for only storing very large files.
Nov 23, 2018
Thanks for your quick reply! I thought (with my limited knowledge about these things) it had something to do with how SSDs operate. I frankly need an external drive to store larger media files (movies, tv episodes) but only for immediate use- that's why I chose smaller external SSD option instead of a large capacity external HDD. I pick a movie or two, or a season of tv show to watch and I just want it to transfer to the external drive as fast as possible. It's not meant to be a backup solution or anything like that. But it seems I might have been better off with a bigger capacity external HDD. Would you say that would better suit my needs or would it be slower than what I currently use? Thanks!

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