On this case if you want to make sure that the Solid State Drive is working properly I suggest you to install the Intel(R) Solid State Drive toolbox and perform a full diagnostic from the following link:
if you do not get any error message then try a different SATA port, update the BIOS and install the latest SATA drivers available for your system.
Here are the specifications for that Intel(R) Solid State Drive so you can confirm the speed and features of that Solid State Drive:
CrystalDiskMark uses incompressible test data as default. Drives with Sandforce controllers will only reach their advertised maximum write speed if you feed them compressible data (which you can configure CrystalDiskMark to use).
The more modern controller design of the 840 pro does not rely on compression to reach its write speeds. But you wouldn't see those numbers with a 128GB 840 pro, although it would still be significantly faster than the 520 120GB.
I have updated drivers, Bios etc and run the toolbox once in a while.Thanks for the reply MBV2000.
The machine runs well with the 520 for everyday use but I move large blocks of data occasionally and it could be my imagination but it seems the 840 does it faster which is important to me.
Oken thanks for the explanation about compressible data etc.