Excellent use for an old phone - secure backup server
I found a cool use for my old Samsung S5 (rooted). I have already been using Dropbox and Google Drive to backup my most important files (from my main linux Desktop) but call me paranoid, I was still concerned about possibility of a remote hack that could in theory wipe out both my main files as well as the linked cloud backups. So.. I had an idea.
Setup termux and a few scripts on an unused S5 to do a daily backup to the phone. S5 has ssh key for the desktop but not the other way around and S5 has no outward facing services running. Each day, it would first rsync ~20 randomly scattered static PDF, DOC, image and source files, compare their hashes to make sure they have not been changed (ransomware encryption detection) and only then rsync the whole backup from the desktop. It then adds it to a local borgbackup repo. I have a separate script that prunes the oldest backups if I ever run out of storage but considering that my daily changes are very small, I can easily have over a year of daily backups even in the phone's built-in storage and I also keep another borgbackup on the microSD card. I am far from being a security expert but to me this seems like a pretty secure setup, not to mention - completely free thanks to termux. Otherwise I would have had to use something like RaspberryPi which is nowadays very much not free.