How much space SHOULD you leave on a 256GB pen drive?

As HWTech and Kurt Lang state… you can fill a removable flash key / pen drive right to its capacity, and the storage limit is what will fit. If you try to overfill it, FAT can get corrupted and confused, but as long as it all fits, it’s all fine.

Cheap removable flash drives don’t have wear leveling or other reliability features, and also don’t have other apps accessing the storage device in parallel as would an SSD on a Mac, and you’re (probably) not booting from them except as part of recovery. And if macOS Recovery fits on your removable flash drive, you’re good.

If you are writing frequently to the same storage (on a removable flash storage device without wear-leveling support) then that part of the flash storage can or will degrade and fail. How long depends on how good that flash is; not easily predictable.

For transient storage, reformat and reload the movie if the flash drive gets corrupted, or replace it when it fails.

If this is your sole backup of some important data, I’d look to have a spare backup device and rotate backups, or maybe use some backup storage elsewhere whether locally, or on a NAS, or in iCloud, or such. Why? Removable flash storage drives (and even SSDs) will usually work great right up until they stop working; they tend to fail hard, not fail relatively slowly as HDDs often do.

But for moving a movie for playback elsewhere? Whatever fits works.

As for AI… AI answers can be right, wrong, outdated, dangerous, or just ordinary spew, and all of that mixed together. Worse, AI around here means we also get to explain why the statistically-generated text approximating the look of an answer provided by an immense advertising and tracking firm is, well, wrong. AI as spectacular at generating text for fluff, for bull, and for management memos, not so good as a technical reference, and often quite hazardous where there is a cost for being wrong. And HWTech covered why this particular text is wrong.

AI is giving you recommendations for SSDs instead of USB sticks. And likely bootable SSDs instead of just data only drives. SSDs already have a small buffer built into them, but it is all about how you use the SSD and even the type of SSD can influence things a bit. For an SSD, if you are constantly writing large amounts of data, then having a bit extra buffer can be advantageous for both performance & minimizing wear.

FYI, the quality of most USB sticks is extremely poor as are their performance. Even name brand USB sticks tend to have issues.

3 GB seems more than sufficient to me. A person has to consider a flash/pen/thumb drive is rarely, if ever used as working space that would be constantly written to with temporary or cache data. You write files, you remove them, and that's about it.

That's exactly what we use one of our thumb drives for. We load it up with shows and movies we want to watch on our UHD Blu-ray player. Once watched, they get deleted.

One drive is reserved for the player since it requires being formatted with a Master Boot Record partition map and ExFAT in order for it to see anything on the drive.