Duplicate Volume Serial Numbers

We recently released a product version, with yearly licenses attached to the machine’s Volume Serial Number.  Now it is called a ‘serial number’, and it seems as meaningless and as random as a UID (mine is 34EE-10A0), so it must be a UID. Right?

Well, not quite. This ID characterizes a volume, not a disk. If you have a partitioned disk, just type at a command prompt  ‘dir c:’ and ‘dir d:’ (or whatever) and watch your partitions’ different VSNs. As the link teaches, the VSN data is part of the partition’s extended boot sector, and is no more then a hash of the partition-creation date & time (i.e., disk formatting date & time).  So, it’s not technically unique – if any two disks are formatted (or partitions created) at the exact same time, they’d have identical VSN. Also – since its only 4 Bytes, the chances of a random hash-duplication are very real.  Just for the sports, if it’s evenly distributed and the world has, say, 1 billion computers,  the chances of duplicate-free distribution of VSN is around 0.187^(1 billion). So there are out there in fact quite a few duplicate VSNs.  But hey – unless you’re Microsoft, such global-scale stuff really shouldn’t trouble you. I mean, c’mon – say you have – what, 1000 clients? 10,000?  make it a hundred-thousand clients. You should never worry about the chance of a duplicate VSN. Now should you?

The real and sad answer, as I recently discovered, is that if you have two clients who use an identical computer model (at least by Dell, but probably true for all other major vendors), the chance of them having identical VSN is exactly ONE.

Dell do not separately format and install every hard drive of the kajillion they deploy. They make some master copy, then deep-copy it around (as us home users do with Acronis, Norton Ghost or whatever). As noted, the VSN is part of the data on the disk, and so is copied as well.

We tried to confirm this officialy with Dell, so far without success. The issue has very sparse web presence too, hence – this post. Hope it helps someone.

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