A metal 128 GB USB on my keychain next to the U2F key
16 GB Ventoy partition with:
Clonezilla (‘deploying’ my system image and backups)
Mint Debian Edition (everything needed to test and recover my Debian systems)
Debian netinstall
Various manuals and reference documents
Portable CrystalDiskInfo and VeraCrypt for Windows
Dumping grounds for files that I intended to transfer between machines, particularly the XP retro gaming rig
An optimistic IF-FOUND.TXT
KeePass
Previously Windows, until once upon a time, I booted into WinRE via Ventoy, got confused between X:, C:, and whatever else, and proceeded to nuke my USB instead of another disk. The Windows installer lived on its own USB happily ever after.
And a LUKS encrypted partition in the remaining space with more documents and a backup of almost all of my photos.