An SSD in a USB 3.0 Enclosure: Part 2 – Transcend SSD340 256Gb + StoreJet 25S3

Having just done a review and teardown of the Transcend SSD340 256Gb SSD, astute readers will note that the product was purchased in a bundle with a Transcend branded USB 3.0 2.5″ enclosure. While the inclusion of the enclosure is generally targeted at users who are looking at an easy way to migrate the data from an internal drive to their SSD (say, laptop users), it opens up a new possibility – turning it into a USB 3.0 SSD.

One of the more popular articles was my former, Am I Crazy? An SSD in a USB 3.0 Enclosure. In that article, I cover the motivations behind creating a USB 3.0 connected SSD and how it pays performance dividends compared to most USB memory key devices. However, I also noted some cautions with unexpected power loss in another article published soon-after.

I’m pleased to report my Kingston SSDnow V300 (Sandforce SF2281 based) SSD in an enclosure is going great – no data loss has been experienced and performance degradation has not been apparent. In general, I have preferred SandForce/LSI based drives for these roles because of their compression in reducing write amplification, their generous over-provision and effective block management even in case of non-TRIM environments.

As the Transcend SSD340 came in a bundle with an enclosure, and I was going to review the enclosure anyway, this would be the ideal circumstance in determining whether the Transcend SSD340 would make a good USB-connected SSD.

Unboxing the Enclosure


The enclosure is the Transcend StoreJet 25S3. It claims to support SATA 6Gb/s connectivity with HDDs and SSDs. It features a one touch backup button and file encryption software (by download). It is 13mm in height, and features a rounded aluminium shell with plastic ends which are secured by two phillips head screws.


The enclosure is covered by a 2 year warranty, and is backwards compatible with USB 2.0. The package includes the enclosure, USB cable, quick start guide and warranty card. The software can be downloaded online.


Also included in the package are some promotional leaflets for other products in their catalogue.


The enclosure itself is a rounded aluminium shell with the logos printed on one side, and nothing printed on the other. A small cutout is made for a window that exposes the activity LED.

DSC_7967 DSC_7966

The front cap has the USB 3.0 micro-B connector, and backup button, with the other end having the batch number and product name/approvals moulded in. Both ends are secured by small philips screws (you will need to supply your own screwdriver).


The bridgeboard PCB utilizes an Asmedia ASM1153E third-generation USB 3.0 to SATA controller. This chip supports SATA III 6Gbit/s connectivity and UASP. There is also a serial EEPROM that holds the configuration for the chip. This is a much nicer chip compared to the first generation Asmedia ASM1051 (from I used in my first attempt which only supports SATA II and no UASP.


The underside seems to show there is a MOSFET switch to disconnect the drive from the bus to comply with USB standby power requirements, and a polyfuse F2 to protect against over-current.

The enclosure was designed for regular height laptop drives (9.5mm), and thus inserting the 7mm SSD without an adapter results in some rattling if shaken hard. Users are advised to pad out the extra space with some paper to prevent movement fatiguing solder joints on the connector.

Performance Testing – Non UASP

USB Attached SCSI Protocol is a new protocol for external devices on the USB 3.0 bus which improves performance by allowing command interleaving. Unfortunately, UASP is only available on USB 3.0 controllers with appropriate drivers, and under Windows 8 or higher. Unfortunately, that means that the vast majority of the computers running Windows 7 is unable to benefit from UASP. The resulting performance is illustrated below.

HDTune Pro Sequential Read


The drive only reaches an average read rate of 260.6Mb/s – something similar to about SATA II performance.

HDTune Pro Sequential Write


Without UASP, the write speed averages 183.4Mb/s – which is also SATA I/II grade performance. This is all due to the overheads imposed by the USB bus so far.



CrystalDiskMark reflects a similar outcome, however, for some reason, the read rate seems to have suffered. The write rate varies significantly, and this may be due to lack of TRIM in combination with DRAM cache. No significant benefits were seen with queued accesses, indicating the lack of UASP.



No corruption was encountered with H2testw, however, the performance degraded somewhat further to 106Mb/s write rate.

Performance Testing – With UASP

As I also have a high performance laptop with Windows 8.1 and USB 3.0 drivers which support UASP (i.e. the Refurbished Asus laptop), I was also able to determine the performance of the drive with UASP and illustrate the performance differential.

HDTune Pro Sequential Read


The read performance has shot up and is now much more comparable to the SATAIII connection. Noting that the USB 3.0 connection is 5Gbit/s, and the SATA III connection is 6Gbit/s, it seems that the average of 430.8Mb/s is right on the money when the ratio is considered.

HDTune Sequential Write


The write performance with UASP mirrors that of the internal performance – very impressive indeed. However, this result was only achieved after a secure erase. Before that, I saw this …


Not believing the degradation, I ran it again.


Once the drive is filled, and all user accessible sectors are filled, the write performance of the drive degrades to the point where it is slower than even most USB keys. This occurs even if the written pattern is 0x00’s as HDTune Pro does – so no amount of “manual” data shredding will restore performance.

SSDScope (Transcend’s SSD Utility) refuses to detect the drive when it’s inside an enclosure, and thus refuses to TRIM or secure erase. Deleting and creating a partition on the drive using Control Panel doesn’t restore performance either – indicating Windows does not perform TRIM on USB connected SSDs.

After a manual disassembly and secure erase, I continued with the next benchmarks.



The impact of UASP is shown in the queued commands having much better throughput than the unqueued benchmark. It does seem like the choice of USB controller may have influenced the 4k result as well, so unqueued requests on this controller (Intel) seems to be slower than Bulk-Only-Transport unqueued requests on the other (NEC/Renesas) controller.



No corruption was found, although it seems that the verification speed was limited by CPU-limitations of the program.


The impact of a lack of overprovisioning on this drive is clear. The Jmicron controller isn’t able to manage the blocks in such a way that once the drive nears (or reaches) complete usage, the write performance degrades unacceptably. It was also determined that TRIM does not work via USB 3.0 enclosure regardless of UASP support, and that manual TRIM is not possible either as SSDScope will not detect the drive inside the enclosure. Only by removing the drive into a desktop connected by a SATA connection can TRIM or secure erase be undertaken to recover full performance (hence, the degradation is not due to write throttling).

Unfortunately, the situation of operating without TRIM and at near full capacity increases the write amplification which further accelerates wearout of the SSD. Operation without TRIM occurs when the SSD is connected through a USB 3.0 case, under operating systems not supporting TRIM or using SATA/RAID controllers with custom drivers, or in a RAID array.

As a result, the Transcend SSD340 is not recommended for use in those environments. I still continue to advocate the use of Sandforce SF2281 based solutions in USB enclosures for this reason.

About lui_gough

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6 Responses to An SSD in a USB 3.0 Enclosure: Part 2 – Transcend SSD340 256Gb + StoreJet 25S3

  1. ginbot86 says:

    I’ve also found that TRIM isn’t passed through the USB-to-SATA converter chip, even ones that support the ATA PASS-THROUGH commands for S.M.A.R.T. and whatnot.

    I use Hard Disk Sentinel to grab disk info and S.M.A.R.T. health data as well. It even tests TRIM functionality, and in the case of a SanDisk Extreme 32GB (essentially a small SSD on a stick) HDS says that TRIM is “supported but disabled”.

    Screenshot of it here:

    There was a utility out there called wiper.exe, but I don’t think it’ll do anything for USB-mounted SSDs; I haven’t gone out to test this, however.

  2. Paul says:

    What enclosure are you using with your Kingston SSDnow V300 ? I did t see it mentioned in the article. I’m thinking of going this route to use as a backup device for photos and important files. Also any suggestions for a good backup software for Windows? On my Mac I use SuperDuper but it does not have a Windows version. Would want a smart copy feature that only syncs changed files. Thanks for the thorough article.

    • lui_gough says:

      My older SSDNow V300 was put into a brand-less enclosure. Ultimately it’s the chipset and firmware that matters in terms of performance – not the brand of the enclosure. The build quality was an issue however, and the port failed, so it was replaced with a Transcend Storejet 25S3 later on.

      I don’t use any synchronization software myself, so I can’t really comment on that unfortunately. I’m sure some research might turn up something.

      As for staying away from Kingston – my opinion is that it really doesn’t matter especially for USB 3.0. You’re unlikely to feel the performance difference between toggle, sync and async. However, if you really care, you can boycott all you like – it doesn’t stop the fact that it’s still a Sandforce based SSD, and it has a very good price, at a time when Sandforce solutions are starting to disappear in favour of cheaper and possibly less stable Silicon Motion, Phison and Jmicron controllers in the value segment.

      – Gough

  3. chelseavanvalkenberg says:

    Supposedly the JMicron JMS578 chipset passes through trim now. I got around this for quite a while now by simply using an ssd which didn’t even support trim, the intel x25-e SLC. It simply powers through the clutter.

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