[LAU] OT: Bash help to check new USB keys.

[Charles Z Henry]

On Wed, Jul 8, 2015 at 1:48 PM, Kaza Kore <dj_kaza at hotmail.com> wrote:
> Date: Wed, 8 Jul 2015 18:37:04 +0000
> From: f.rech at yahoo.fr
> To: dj_kaza at hotmail.com
> Subject: Re: [LAU] OT: Bash help to check new USB keys.
>
> It's not a full testing of a flash device I'm worried about, especially as
> they are new, I just want to know they really are the size they report as
> being...
>
> Dale.
>
> Probably GParted will tell you that,
> HTH,
> Fred
>
> Except the likes of GParted, df, udisks etc are exactly what I don't trust!
> I have read too many reports of people buying say a 128GB usb drive, copying
> loads of data onto it and later discovering everything after say 8 or 16GB
> isn't really there! Somehow they fake the part (ToC? MBR?) which the
> computer reads the size of the drive from and seems (at least in Doze-land,
> where most of these reports are from, but then again so is most the computer
> world) that the system even reports having written the files correctly and
> they show up in the table of contents and in your file explorer as you would
> expect. Hence I want to actually write data until the drive is near full and
> do an md5sum on the files I have written. It's only a single write of the
> few thousand they should be usable for and I plan to use them predominantly
> as back-up storage so I don't envisage lots of erase and re-writes over
> their lifetime. Basic drive integrity isn't a worry. Being sold dodgy, fake
> components which report the wrong size is.
>
> I did wonder if doing something as simple as a full (rather than Quick)
> format to the likes of ext4 might catch out something like this too...
>
> Dale.

Programmers like to report size in gibi bytes, and manufacturers
almost always use giga bytes.
https://en.wikipedia.org/wiki/Gibibyte

There's about a 7.5% difference between the units.  So, the units can
make it look like you're losing 8GB off a 128GB disk, or 16GB off a
256GB disk.

I have to explain this to people all the time... no.... the software
is wrong according to definitions, so you have to add/subtract x% to
get your command right.

So, do some math and see if you can explain the discrepancies with units.

Chuck