[isr]

There are many which could fit your description. Tiny core, puppy linux varients (voidpup, with the void pkg manager, is pretty nice), etc.

However, I think the nicest put together ones are fatdog64 & porteus.

fatdog64 is built from scratch (basically, "Beyond Linux From Scratch"), but uses a lot of slackware tooling & ethos, so its quite compatible with slack pkgs & slackbuilds. If you dive deeper, its actually got a lovely cohesive design, behind the minimalistic gui which IMHO does it a huge disservice.

Eg: it has scripts to run containerised or UML'ised versions of itself, from within itself. Down to optionally having a nested X session with full gui.

Porteus (or the more bleeding edge Porteux variant) has more conventional look & feel out of the box.

Both Fatdog & Porteus(x) have much more flexible initrd systems than tiny core/puppy, where you can copy configs in from a static location (rather like alpine's overlay tarball), or bind mount stuff in. Critically, this is all before the main rootfs goes live, so you can affect how various disks & services are handled on a machine-by-machine basis, if needed.

(plus they're not restricted to the root-only approach which puppy needlessly adheres to religiously)

Plus, these were the original IMMUTABLE distro's, long before that was adopted by some distro-giants.

In other words, with a little thought, you have TONS of flexibility. Including the scenario you mentioned.

To get a flavour, have a quick look over

- [the fatdog faq](https://distro.ibiblio.org/fatdog/web/faqs/faq.html)

- [this old blog post] (https://www.lightofdawn.org/wiki/wiki.cgi/FatdogIsVersatile)

PS: to install either, it can be as simple as setting up a bootloader (which IMHO ought to be a distro agnostic task anyway), and copying over ... 2 files (by default, fatdog shoves its main rootfs squashfs module INSIDE the initrd). With porteus, you will have a fully setup xfce, cinnamon or gnome (yes) setup with ... 4 files.

[jauntywundrkind]

Edit: Puppy Linux does seem to boot to ram! Huzzah. Interesting thread that goes over some behaviors available: https://oldforum.puppylinux.com/viewtopic.php?t=36508

The unique problem here is that while many of these are designed to run nicely from USB drive, I don't get the impression that many will let you remove the USB drive after boot.

FatDog talks about running from a RAM layer, which is cool. But I believe even this is still a RW layer atop the underlying root which needs to remain mounted.

[isr]

Nope, puppy (all variants), fatdog, and porteus (all variants, including porteux & nemesis - which is arch based) can all boot in a mode where you can unplug/unmount the storage device from which it booted.

(as can tiny core, alpine, etc)