SOLUTION FOUND
You need not read any further, unless you're curious: @bobafetthotmail created a Quick Start for Adding a USB Drive to address this issue.
Many useful programs such as squid, Samba, YAMon, etc. require the use of external storage. The easiest way to do this on modern routers is to connect a USB stick (or USB hard drive) to the router.
In my case, I "just want to connect my new 4GByte USB stick to my LEDE router" so I can use some of the software named above.
It would be an enormous service to the entire LEDE community for someone who knows how all this stuff works to create a simple HOWTO page on the www.lede-project.org wiki.
Requirements:
- LEDE 17.01.0 or newer firmware
- Router with at least 32MBytes of RAM and 64MBytes Flash (no swap needed, plenty of space for packages)
- USB device of unknown filesystem and sufficient capacity (say, 1Gbyte or more)
- Format the new storage for ext4
- Mount the new storage in /opt
- Written for a novice (that is, "Just do step1, step2, step3... and you're done")
Non-requirements:
The "USB Storage" HOWTO needs to focus on the items above, and doesn't have to address any of the factors below. In fact, the HOWTO shouldn't talk about these - they add complexity with no value to the issue at hand:
- Don't worry about pre-LEDE software. We're all using LEDE. All the existing guides on the OpenWrt site have long sections about ancient software. There's no way to evaluate which of these apply to modern firmware.
- Don't worry about filesystems that aren't ext4. We just want to wipe the data on the stick, and then use it with LEDE. Talking about other formats simply clouds the issue.
- Don't worry about a LuCI package. It would be great to have a GUI that does this, but requiring SSH is fine.
- Don't worry about options. We've already chosen LEDE firmware and a particular USB stick. We just want to install and use it, and don't care about alternatives. (If there are choices, the procedure should choose the sensible defaults that always work.)
Many thanks!