SSD not reaching advertised speed
SEQ1M maxes at 527MB/s which is pretty close.
Unless the disk is completely empty and not used at all by your system then this is pretty much what you'd expect to see. 550MB/s is the theoretical maximum and assumes that the testing program is the only thing running and that the link is otherwise "perfect".
There is almost always some overhead, either in the device chain between your CPU and the SSD, or in something else using the SSD.
You are within about 5% of the advertised drive speed. There is nothing wrong with the drive. Advertised speeds are "up to" the given speed, not "will always".
Sandisk themselves state the following in the disclosures section of their site
Based on internal testing; performance may be lower depending on drive capacity, host device, OS and application
Unless you can replicate their exact hardware, test procedure, OS and so on then "up to" is about as good as you're going to get.
You will need to install it on a Linux system as a second drive, make sure it is totally empty and formatted as EXT4, then re-run your test using the "dd" command in Linux.
Under those conditions you should find the drive is actually slightly faster than the stated performance numbers from Sandisk, which get rounded down to the nearest 10MB.
The Windows NTFS filesystem adds a little overhead, as does using the drive as a boot drive, which it looks like is what you're doing. Windows will constantly read and write tiny amounts from the drive it boots from while you are using it. Both of these factors will reduce the numbers in your test slightly.
The advertised speed is displayed as up to 500mb/s. There could be multiple reasons why it's slower then the advertised speed like
- need to update the firmware
- AHCI mode is disabled
- wrong SATA port, you need to use the SATA 3 port(if it's below 300mb/s)
- optimize the drivers
- confirm "trim" is running
You can find other things you can try here
If this doesn't bump the speed up, you can contact the manufacturer but they probably won't replace it due to this "up to" rule.