On a sunny day (Wed, 19 Sep 2018 09:04:14 +0100) it happened "NY"
 wrote in :

>I've got a USB hard drive (Intenso 320GB 2.5") which is plugged into my Pi.
>It auto-mounts to /media/pi/ but I umount it from there and
>manually mount it to a folder /mnt/recordings where it is used as the
>destination for TV recordings made by TVheadend.
>
>I've noticed that at random times (not related to movement of the USB
>connection between drive and the Pi), the drive disconnects itself. The
>first I know is when I see a pop-up dialogue "Removable media is inserted -
>Open in File Manager". It remounts to /media/pi/ - always the same ID.
>However I notice that the raw device /dev/sda and sda1 changes to
>/dev/sdb(1) and then sdc(1), and so on for each successive disconnection and
>reconnection.
>
>I was going to reconfigure TVheadend to refer to the /media/pi/ mount
>point, but there's something that prevents TVHeadend working in this case,
>even though I can "ls" the files on the drive and the mount point has 777
>permissions.
>
>Any suggestions? The drive is formatted as NTFS. Could that be relevant:
>does NTFS have some sort of auto-dismount capability.
>
>I'm about to try another drive to see if that fails in the same way. I
>didn't notice a memory stick (formatted as exFAT) fail in this way, even
>though it appears as the same raw device /dev/sda(1).
>
>The drive has been used for ages on various Windows PCs without any signs of
>auto-dismounting.

Looks like a power failure to the drive to me,
could be caused by a bad USB connector or power supply not strong enough.
You can check that when if it changes from sdb1 to sdc1 by typing
 mount
to see when it says sdc1 if sdb1 is still mounted,
that would mean it is not properly un-mounted, check cable... power supply.

I have had a _lot_ of bad USB cable incidents all over the place.
USB drives take a lot of power, current on the USB.

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