Hi everyone,
I have a Lenovo V330 - 151GM Thinkcentre Desktop with a 500Gb HDD running Linux Mint 20.2.
I have been using the following setup for a number of years now ever since I moved from WinXP to Linux Ubuntu probably in about 2015. On this HDD I have two partitions, /dev/sda5 for the system (92Gb) and dev/sda6 (374Gb) for my private files. Both of these partitions are enclosed in an extended partition /dev/sda2 (466Gb). There is no swap file partition. See attached images. For some time now there has been an occasional (1 to 2 times per month) mounting problem after boot up, when my personal partition (sda6....RRG Documents1) does not automount correctly and is not found by Caja File Browser. This problem is generally fixed by opening Disks, then unmounting sda6 and then remounting it, when it then mounts correctly and can be found by Caja. However this does not always work and the system then requires a reboot after which everything is OK. Although it is not really a hassle, I would like to know what could be causing it and how I can fix it. Any ideas? Thanks.

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That sounds like a fileystem corruption. Which might be a disk on its way out - or might just be passing neutrinos.
What do the logs say ?.

Does the Disks app give you an option to check the health of the drive ?.

When the problem first manifested itself, my first thought was that the disk was on it's way out (disk is an old seagate). However this problem started before Covid (new area?) but the disk passes SMART and displays no further signs of deterioration. I make a twice weekly snapshot on two different portable drives using Clonezilla, and am now not totally convinced that it is the HDD. You mention Logs, please tell me which logs and how to display them.

Ok, good you know about SMART - and good to know it passed. Shutting down the system incorrectly for example can cause filesystem corruption - the reboot forces a fsck and (hopefully) fixes it.
My Mint 20.1 test laptop has a persistent systemd journal - not sure if that was the default or I set it up. What does "journalctl --list-boots" reveal ?. It uses less as a pager, so q to exit the list.
If you know when you had the problem, you can use the first number to pull up the logs for that boot as in

Result of running code in terminal:
Specifying boot ID or boot offset has no effect, no persistent journal was found

I have specified a forceed fsck after every 2nd boot. Can't really say when the problem started,I think it was some time in early 2020 so I doubt that the logs would still be there.

You need to read all of my prior post.
I can't help you with what logs Mint uses if the journal is non-persistent, you will have to wait for another user. Any occurrence should give you relevant messages, not just the first. Persistent logs have many advantages - on my main Fedora system they go back over 10 years; I search them regularly.

My apologies. You will have to forgive me as I am not really clued up as far as code is concerned. Running 'journalctl --list-boots' results in:

rrg@rrg-V330-15IGM:~$ journalctl --list-boots
0 f79e000559ea43668b4d63fce907da8d Sat 2021-08-28 06:43:28 SAST—Sat 2021-08-28>
lines 1-1/1 (END)

This is repeated many times with only the time changing. In between '0 f79.......2021-08-28' and 'lines 1-1/1 (END)' are a lot of '~' one above the other on the LH side. See attached image.

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According to your attached images /dev/sda6 is either being mounted at /media/rrg/RRG Documents1 or /run/timeshift/backup.

How is it it being mounted via /etc/fstab?

You are correct as to the two mounting points, as to how it is being mounted, I haven't a clue, how can I find out? I store the Timeshift snapshots on /media/rrg/RRG Documents1, but cannot explain why sda6 would sometimes be mounted at /run/timeshift/backup, when it should be (and 99% of the time is) mounted on /media/rrg/RRG Documents1.

Is /dev/sda6 being mounted using /etc/fstab or automatically by the system? I would guess the latter.

Check /etc/fstab to see if there is a line for /dev/sda6 which might be listed only using its UUID.

rg@rrg-V330-15IGM:~$ sudo /etc/fstab
[sudo] password for rrg:
sudo: /etc/fstab: command not found
rrg@rrg-V330-15IGM:~$

Perhaps a different code is required to check fstab?

/etc/fstab is a text file not a command

cat /etc/fstab

Thanks.

rrg@rrg-V330-15IGM:~$ cat /etc/fstab
<file system> <mount point> <type> <options> <dump> <pass>

UUID=c0db6a0a-4e51-4697-b227-3c5cb51cc59e / ext4 errors=remount-ro 0 1
UUID=1840b07a-b7b8-451b-a4ff-aaa944b9b1bf none swap sw 0 0
/swapfile none swap sw 0 0

rrg@rrg-V330-15IGM:~$

Sorry, also post the output of the command
lsblk -f

rrg@rrg-V330-15IGM:~$ lsblk -f
NAME FSTYPE LABEL UUID FSAVAIL FSUSE% MOUNTPOINT
loop0
squash 0 100% /snap/core
loop1
squash 0 100% /snap/core
loop2
squash 0 100% /snap/core
loop3
squash 0 100% /snap/core
loop4
squash 0 100% /snap/gnom
loop5
squash 0 100% /snap/gnom
loop6
squash 0 100% /snap/gtk-
loop7
squash 0 100% /snap/huns
loop8
squash 0 100% /snap/gtk-
loop9
squash 0 100% /snap/mp3g
loop10
squash 0 100% /snap/snap
loop11
squash 0 100% /snap/snap
loop12
squash 0 100% /snap/mp3g
loop13
squash 0 100% /snap/vide
loop14
squash 0 100% /snap/vide
sda
├─sda2
│
├─sda5
│ ext4 c0db6a0a-4e51-4697-b227-3c5cb51cc59e 36.9G 54% /
└─sda6
ext4 RRG Documents1
8e3c148c-a196-4889-adc1-5d3cff192833 173G 48% /run/times
sr0
rrg@rrg-V330-15IGM:~$