Can two USB flash drives conflict with each other?

[bob prohaska]

On Sat, Mar 03, 2018 at 12:32:27AM +0000, tech-lists wrote:
> On 02/03/2018 23:13, bob prohaska wrote:
> > The obvious solution is "don't do that!", but if somebody can offer
> > a more insightful explanation I'd be grateful. Using two USB flash
> > drives simultaneously would be very useful.
> 
> I've found [this was a year ago, maybe two] that if I had two usb sticks
> plugged in that sometimes they'd be detected in reverse order to what I
> expected.
> 
> What I mean is that sometimes the device called /dev/da0 and the one
> called /dev/da1 would swap on reboot. I suppose it would depend on which
> one woke up first. So if I had made /dev/da0p1, allocated it as swap,
> /dev/da0p1 as data, perhaps put the ports tree there, /dev/da1p1 as
> data, perhaps used the entire device for data, sometimes it'd boot, look
> at /dev/da0 which was /dev/da1 previously, not seen swap, and complained
> loudly.
> 
> I think there is a way to wire device identities to names but it might
> need GPT rather than MBR as a partitioning scheme. I worked around it by
> labelling one of the usb sticks with sticky tape and ensuring it wasn't
> plugged in before the other one when rebooting.
>
 
On the first try I plugged the second USB drive into a running machine,
producing the errors reported. It's not obvious how a _second_ device
can "unseat" one that is already represented in /dev/....

On a later try I plugged the second USB flash device in and powered
the Pi3 up, whence the kernel got confused over which was which. That
makes slightly more sense. I think that might be fixable with labels
in /etc/fstab. In my case the second drive was labeled much like the
first, so it couldn't help.

Somewhere I got the idea USB flash devices had a unique serial number,
or equivalent, so that more than one could co-exist on a host. 
Is this notion mistaken?

Thanks for reading,

bob prohaska