In case there are two or more admins in ubuntu, can one admin prevent another admin from accessing his/her data ?
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And anyone with physical access can change anything on a system. They do not even have to be an admin; they just need to be able to reboot.
The only way to prevent others to see your data is by storing it on another media (usb stick or on-line). And to lower the risk of exposure you always need to unmount whatever media the data is stored on.
Another admin can view your data when you have the data visible in your system. Mounting an encrypted volume as "root" is not enough; another "root" user will be seen as owner of the data.
Someone that can reboot the system can also reset the password of an encrypted volume. Yes, you would know someone accessed it but ... the whole idea was to prevent access.