Find the working location
A large media disk does not guarantee space in the filesystem Jellyfin uses for temporary output. Identify the configured working path, its host or container mapping and its available capacity. Keep that role separate from the original media and the server database in your notes.
Observe one representative job
If you conduct a local check, use an authorised sample and record the client, media characteristics and whether transcoding actually occurs. Compare free space before, during and after the session. A direct-play session is not evidence for the temporary-space demand of a different transcoding workload.
Do not turn one observation into a universal number of simultaneous viewers. Duration, output settings and cleanup behaviour matter. No playback measurements were performed for this article, so it provides a review method rather than a recommended reserve value.
Keep a response for low space
Record how the administrator identifies the active job and uses the documented task or cleanup controls for the installed version. Avoid deleting unknown live working files as a first response. Revisit the budget after changing the temporary path, client mix or expected session length.
Questions answered
Can free space on my media disk answer this check?
Only if it is actually the filesystem used for the relevant working output. Verify the configured path.
Does one successful direct-play video establish transcoding capacity?
No. Observe the workload that creates the temporary output you need to budget.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.