Upgrade can be stopped from running amuck

This topic has been troublesome for so long that the original topic has been closed:
>> https://trisquel.info/en/forum/apt-get-upgrade-runs-amuck-trisquel-thumb-drive

Nevertheless, I persevered and can now report limited success.

Here's what can be done:

1. Start with a good USB3.0 flash drive of any convenient size; I chose 16GB.
2. Reformat all of it with Gparted to fat32.
3. Shrink the original partition to a little more than 4GB (how much more than 4GB is an open question).
4. Add a new ext4 partition to the remainder of the space in the USB stick.
5. Use Startup Disk Creator to install Trisquel 7 (I chose the trisquel_7.0_amd64.iso) on the 4GB partition.
6. In Synaptic Package Manager, mark every software support package for removal except one (in my case: USA english).
7. Run Software Updater.
8. Boot up into the Trisquel 7 OS that is now in your USB flash drive, and add the desired files to the ext4 Data partition.

Unfortunately, this process leaves next to no room for additional applications in the persistence file, which is a counterproductive marketing move for popularizing Trisquel 7 to the computing community.

Additional comments:

Step 6 is a tedious task; over 300 packages will have to be selected and marked essentially one at a time, which takes about an hour, but the eventual result will be that about 75,000 fewer files will have to be written in that limited-size 4GB partition, particularly to the even more limited persistence file.

Do not chose a USB2.0 flash drive ... it will be slower than anything you could possible imagine. My early attempts took 18 hours for the Trisquel installation process, and my Data partition's browsing performance was glacially slow. In a USB3.0 flash drive the browsing speed is indistinguishable from the internal HDD's speeds, and the installation and upgrade are pleasantly quick.

When I first did this about a year ago, I was able to run my laptop for several months from a frequently updated/upgraded Trisquel 7 on a 4GB USB flash drive that only ran out of space when I installed way too many applications and then had to be reprogrammed ... until it physically wore out and fell apart. Since then, Trisquel has grown from version 39 to version 79, and now that 4GB of space is way too small, but the Startup Disk Creator and casper (?) no longer allow sufficient storage space.

My 16GB Trisquel 7 flash drive is now a nice marketing tool for showing how well this fine OS can function, especially with the Data files that I am passing around to my family & friends, but there are only about 50MB of persistence file space into which to shoehorn a couple of application files.

I tried this process with a 500GB USB-connected IDE HDD, and the "Live USB" Trisquel 7 installation runs out of space on that one, too, even with a 30G fat32 primary partition as the starting place.