Tvheadend

I'm using TVH 4.2.6 on a Raspberry Pi 3B+ running Raspbian Stretch. Rather than write to the SD card, I write to a pen drive (I'm planning to write to a USB hard drive when I get a powered USB hub).

I found that auto-mounting /dev/sda1 as /mnt/rec was unreliable because sometimes the pen drive would show up as /dev/sdb1, c1 or whatever. So instead I mount by UUID (determined by the blkid command) and have added a line "UUID=1CAB-16D4 /mnt/rec exfat defaults,errors=remount-ro 0 1" to /etc/fstab.

I then set TVH's Configuration | Recording | Recording system path: to /mnt/rec

This works perfectly, surviving any number of reboots, until one day a recording fails with "file missing". From the terminal app, /mnt/rec is still visible and still has the contents that I would expect (existing recordings). I've also Samba-shared /mnt/rec by adding the relevant lines to /etc/samba/smb.conf and restarting Samba. When the recording fails, I can still access the folder remotely by Samba (eg from a Windows PC). That makes me wonder whether the fact that the folder /mnt/rec is an external auto-mounted device may be a red herring.

For the time being I've modified Recording system path: to /home/pi/Videos and Samba-shared that instead, so I'll see if it works any better writing to an ext4 / partition on the SD card.

Anyone got any suggestions as to how I can troubleshoot the problem with using an auto-mounted pen drive as the recording location? Is it likely that there are intermittent problems with the pen drive because it is connected to the Pi rather than a powered hub? I've confirmed that the voltage on the Pi's supply pins is about 5.1 V even with pen drive and two DVB-T decoders plugged in, so it's probably not an under-voltage problem.