is there a way to get to them while mounted?
Not that I know of. You could mount the existing storage somewhere else (another mountpoint) if you need to transfer anything to or from that. Or use a Live session. 
To umount the external drive:
$ sudo umount /mnt
Check to make sure the external drive has been unmounted from /mnt
$ df -h
Now change directory to /mnt
$ cd /mnt
Run this to make sure you are now in the /mnt directory
$ pwd
It should show this now:
/mnt
Warning! Make VERY sure you are in /mnt before issuing the next command, it will remove every file from your current location and traverse every directory removing those files too.
$ rm -fr *
If you are in / when you run that command, it will remove every file on your SD.
And finally, remount the external drive or reboot.
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If you know what processes are running that access the drive, you will need to stop them. Hopefully you know, hunting those down can be difficult.
There is a command “lsof” which can tell you which files are open on a filesystem and the process that is accessing those files.
$ sudo lsof /mnt
It is far better to properly stop the processes. Also, if you changed directory to /mnt (ie. cd /mnt) this too counts as a process and will prevent the umount, and tell you the target is busy.
mount --bind / /mnt seemingly unmounted /mnt and the drive is no longer showing up in df -h
Got it! I found the old backup data that was taking up space - back down to 3.7G now!
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setting boot partition as read-only can prevent this - do you know any easy way to do that? i’d need to disable it only to update system/server packages
Have you checked man fstab?
There’s an example in this forum: Howto mount drive as read-only with fstab - Ask Ubuntu
There’s also this, which is more raspberry pi relevant:
yeah I found a guide for Raspbian. Looking at my fstab:
LABEL=BOOT_MNJRO /boot vfat defaults 0 0
thats obviously main filesystem partition, and I would change that to:
LABEL=BOOT_MNJRO /boot vfat ro,suid,dev,exec,auto,nouser,async 0 0
this is great so far but it would just affect mount point /boot:
/dev/mmcblk2p1 214M 53M 161M 25% /boot
the partition that had the hidden data is: /dev/mmcblk2p2 15G 3.7G 9.8G 28% / but there is no mount point, so this wont be affected by the config above right?
Yes, but you said you wanted to make the boot partition Read-Only:
so changing the entry to: LABEL=BOOT_MNJRO /boot vfat ro,suid,dev,exec,auto,nouser,async 0 0 would do the trick then?
I don’t know about the others, but the ro one should be there atleast.
i just replaced default with all the default settings individually and changed rw to ro
would I still be able to update using sudo/root or will I have to change fstab back to rw before updating system?
No, no… setting /boot as read only will only serve to mess up your updates when you forget to remount it R/W. What I think you are wanting is to set the root filesystem / to be read only (to protect /mnt from being written to)… it can be done I suppose, but you will be greatly limited in the software you can run. Both /etc and /var are a real issue for having a R/O / filesystem.
I suppose you could make 2 extra partitions and mount them on /etc and /var but this will be headache, as partition size will matter and /var can grow unexpectedly. And with a MBR SD (limited to 4 primary partitions), you will have to make extended partitions.
So you will end up with a complex and fragile setup that is not easily “fixed” by simply making a new SD card from an image. You might be better off waiting for the UEFI firmware to be production ready for more complex filesystem configurations. But I do not know a timeframe of when this will happen.
There is something to be said for the simplicity of the RPi4 and SD cards, given the current immature status. I enjoy these types of complex configuration challenges myself, but not everyone else will.
yes, the root filesystem.
the main reason for wanting read-only is to prevent corruption in case of improper shutdown - the writing to /mnt was a one-off mistake and not an issue in future.
I got the idea from this thread: Raspbian with Read-only Root - Raspberry Pi Forums
im using a RockPi4 booting from eMMC, is there a way I can benefit from what they are doing in the thread above?
I am working on network booting a good number of RPi4 and I would very much like a R/O root filesystem, so they could all share a single root filesystem. So this is a work in progress for me as well. If I am successful in making a R/O filesystem that works for a general use desktop, I will for sure post it in my thread on network booting the RPi4.
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