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storage devices with same marketing size having different number of blocks

On 05/26/2017 03:16 PM, Fungi4All wrote:
Here is a surprising experience. Recently I asked the store to give me a few
of the cheapest and nastiest usb sticks, 8Gb if they had. The usb3 16Gb
were only a buck more. So I got identical sitting side by side on the rack
Intenso 16Gb usb3. I installed a system with 3 partitions on the one, booted
configured and wanted to make a backup with settings for network for a
seconf machine. So I used dd if=sdc of=sdd
I go in and check and there were a whole bunch of Mb more on the second
drive with no partition while the partitions that copied seemed identical.
So I wonder how long it would have taken me to figure out the error caused
by dd if I had randomly used the second as first and the first as second.
I think they are like uuids, there are no two identical usb drives ever.

It is common for storage devices with the same marketing size to have different numbers of blocks. Usually the same make and model of devices within a given production run (e.g. hardware revision) have the same number of blocks, but not necessarily so for different production runs (I have SanDisk Ultra Fit drives like this).

If imaging is in your bag of tricks, one work-around is under-provisioning -- e.g. partition new devices with less than 100% of the available blocks; leaving unused blocks at the end. (Choosing the "right" number of blocks to leave unused is an exercise left for the reader.)

AFAIK USB flash drives don't have TRIM or secure erase, but there may be controllers/ firmware smart enough to recognize zero blocks and optimize their use (?).


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