Since at least v1.0.0 Petitboot has used device-mapper snapshots to avoid mounting block devices directly. Primarily this is so Petitboot can mount disks and potentially perform filesystem recovery without worrying about messing it up and corrupting a host's boot partition - all changes happen to the snapshot in memory without affecting the actual device.

This of course gets in the way if you actually do want to make changes to a block device. Petitboot will allow certain bootloader scripts to make changes to disks if configured (eg, grubenv updates), but if you manually make changes you would need to know the special sequence of dmsetup commands to merge the snapshots back to disk. This is particulary annoying if you're trying to copy logs to a USB device!

Depending on how recent a version of Petitboot you're running, there are two ways of making sure your changes persist:

Before v1.2.2

If you really need to save changes from within Petitboot, the most straightforward way is to disable snapshots. Drop to the shell and enter

nvram --update-config petitboot,snapshots?=false
reboot

Once you have rebooted you can remount the device as read-write and modify it as normal.

After v1.2.2

To make this easier while keeping the benefit of snapshots, v1.2.2 introduces a new user-event that will merge snapshots on demand. For example:

mount -o remount,rw /var/petitboot/mnt/dev/sda2
cp /var/log/messages /var/petitboot/mnt/dev/sda2/
pb-event sync@sda2

After calling pb-event sync@yourdevice, Petitboot will remount the device back to read-only and merge the current snapshot differences back to disk. You can also run pb-event sync@all to sync all existing snapshots if desired.