13
Aug
2015

The dodgy micro-SD card: The Final Chapter

So I have been using a somewhat dodgy micro-SD card I bought from Rakuten (more on this here and here):

I have decided to do a complete analysis of what the card actually is. To Ubuntu, it is reported as so:

Screenshot_2015-08-11_14-57-32

“17GB”. Hmmmm. When I was using this for my Raspberry Pi 2, I partitioned it such that I only used half of the drive’s space (about 8GB) and it seemed to work just fine. So, lets get to the bottom of this.

I have partitioned the full space as ext4, and I am using a program called F3 (Fight Fake Flash) to test the drive. You can get F3 from here.

I compiled the software just by typing make into the unzipped directory, and ran the software with the following command:

./f3write /<wherever you drive is mounted>

This starts off the write phase of the testing. This fills the drive up at the filesystem level with test files, and does not overwrite existing files, so make sure you start with a blank drive when using this method. You can do this test at the block level using f3probe, but this is still experimental and not recommended.

This is the output I got for f3write:

./f3write /media/john/…/
Free space: 15.22 GB
Creating file 1.h2w … OK!
Creating file 2.h2w … OK!
Creating file 3.h2w … 14.90% — 2.45 MB/s — 57:37

And now we wait for the drive to fill up! This looks like it will take about an hour. Fake drives always report success on writing data, even though it could be just throwing data away when it reaches the actual physical limit of the internal storage. Also, this is meant to be a “Class 10” drive. 2.45MB/s is not “Class 10”. Its not even Class 6 or Class 4. Already smelling a rat here…

This is what you see when it has finished:

Creating file 14.h2w … OK!
Free space: 815.95 MB
Average writing speed: 2.64 MB/s

So, the test files have been written. Now, its time to verify, using f3read:

./f3read /<wherever you drive is mounted>

And you should see the files being analysed. Look in the corrupted column to see when these test files become corrupted!

It is finally time to see what this dodgy microSD card really is. Here is the output:

Screenshot_2015-08-11_17-12-15

Well well well. This microSD card only has 5.36GB of usable space. How that works I do not know, but I do know one thing. This card is going straight in the bin. This made me think – I had set the partitioning to use 8GB, when clearly this wasn’t small enough to prevent data corruption. My Raspberry Pi 2 seemed to work just fine, but it was probably because the Pi wasn’t using more than a couple of GB of space. If it went over 5.36GB (or wrote something into the corrupted area), then it would have been game over!

So make sure you get decent SD cards (or any type of flash storage) from a reputable manufacturer. If a 1TB USB drive for £20 seems too good to be true, it probably is.

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