Re: Updating Persistent USB Flash Drive
Bootable iso on USB is intended for testing Ubuntu on systems, installing it, or using it for rescue/repairs. Things that happen during boot are on a fixed squashfs file system that boots before persistent data is mounted.
While you can install programs that you launch after booting, and store certain system settings (timezone, wireless security, additional users) in persistent data, you cannot install or update things like kernel, video drivers, etc. that initially happen during boot. It is also totally insecure in that it boots without a password to a user (ubuntu) who is effectively admin.
So if you want to use it securely regularly and want to be able to update it, you should really do a regular install (8 GB minimum recommended). However, for the partitioning portion of install, make sure that you select "Other" to do the partitioning yourself, and select from the drop down list there to put grub on the mbr of your USB flash drive (not worded that way, but select the proper drive).
i5 650 3.2 GHz, 8 GB, nvidia GTX 1060, 32" 1080p & assorted older computers