Re: Password protect portable external hdd
"ubuntu file format" does not "work both ways". Ubuntu can read/write it; Windows can not. If you want a data partition to be shareable between Linux and Windows, you would do better to use NTFS.
As to encryption, if I had a dollar for every post I've seen over the years from folks who encrypted their stuff only to have something unexpected occur -- and then lose access to all their stuff -- I would be RICH now.
The BEST protection against data compromise is what you're already proposing -- keep the data in a physical location that is external to the PCs in use. Once someone gains physical access to the encrypted drive, it's only a matter of time until they get access to the data. In my personal opinion, encrypting the external drive is only going to cause you problems in the long run.
Ubuntu 16.10 Mate, Mint 18 Mate; MS Win 8.1, MS Win10 Pro.
Will not respond to PM requests for support -- use the forums.