The disk that fills up with frames
How to measure free space on a Mac before a capture day, read System Data and purgeable space, and decide which files may go without losing footage.
A capture day needs roughly three to five times the size of the raw frame set in free space before the first card is offloaded: a 64 GB card of 24 MP raws plus proxies and a working project file will not fit on a drive with 40 GB free. Check the number the night before, not on the morning of the flight. If the figure is below that margin, the decision is not whether to clean, but what may go.
What number should you read before a capture day?
Open the Apple menu, then System Settings, then General, then Storage. The bar at the top reports available space, and the categories below it report how the rest is allocated. Treat the top figure as the only one that matters for planning. A 1 TB internal SSD that reports 180 GB available has 180 GB for the offload, the proxies, the cache written by the editing application, and the temporary files the operating system creates during the copy.
For a second opinion, open Terminal and run `df -h /`. The line for the root volume gives Size, Used, Avail and Capacity. On APFS the Avail column can differ from what Finder shows because containers share space between volumes, and because local snapshots occupy blocks that are not counted as free until they are thinned. Run `du -sh` on the folder that holds the project if you want to know what the last shoot actually consumed, then multiply by the number of cards planned for the next one.
Planning also means knowing what the machine will do on its own. macOS writes local Time Machine snapshots when the drive is not connected to its backup target, and those snapshots hold deleted blocks for up to 24 hours. A drive that showed 200 GB free yesterday can show 120 GB today with no new files on it. The practical habit is to measure twice: once when the cards are formatted, and again the evening before the flight.
A fuller treatment of the same problem, including how the System Data category is assembled and what purgeable space means, is set out in the guides at measuring free space on a Mac, which cover the measurement step before any deletion begins.
Which files may go, and in what order?
The order matters more than the tool. Delete copies that exist elsewhere first, then regenerable data, then anything that would be expensive to recreate.
Start with duplicate offloads. A card copied to the internal drive and to an external drive, then verified, leaves the internal copy redundant. Verify the external copy by checksum or by opening a sample of frames before the internal one goes.
Next, caches. Editing applications, browsers and cloud clients write caches that rebuild on demand. Removing a cache costs time on the next launch, not data. Do not remove the cache folder of an application while it is running, and do not remove the application support folder that holds project databases.
Then render files and proxies. Proxy media for a finished edit can be regenerated from the originals in minutes to hours depending on length. If the project is delivered and archived, the proxies are the cheapest large item on the drive.
Last, originals. A raw frame set that exists on one drive only is not a candidate for deletion. Move it to an external volume instead, and confirm the copy before removing the source. The same applies to the project file and its sidecar data.
How do you read System Data and purgeable space?
System Data is a catch-all category in the Storage pane. It collects caches, logs, virtual memory swap files, application support data, and the local snapshots mentioned above. A large System Data figure is not by itself a fault, and it is not a folder you can open and empty.
Purgeable space is space the system considers reclaimable on demand: it includes local snapshots, some caches, and files stored in iCloud that are not currently downloaded. Finder may count purgeable space as available, while `df` reports it as used. That gap explains most of the disagreement between the Storage bar and the Terminal output.
To see local snapshots, run `tmutil listlocalsnapshots /`. To thin them, `tmutil thinlocalsnapshots / 10000000000 4` asks the system to free about 10 GB with an urgency of 4. These commands change system state; read the manual page before running them, and never run them during a copy.
Should you move libraries to an external drive?
For a capture workflow, yes, with conditions. Photo and video libraries are the largest single items on most editing Macs, and they are also the easiest to relocate because the applications support it.
Move the library file and its referenced originals together, keep the external volume connected and mounted before the application launches, and avoid exFAT for anything that will hold a library database. APFS or HFS+ on the external drive is the safer choice. A USB 3.2 Gen 2 connection at 10 Gbps is adequate for offload and for proxy editing; direct editing of 4K or higher source media over the same link will show dropped frames.
Keep the internal drive for the working project, the cache and the operating system. A common split is 200 to 300 GB of internal free space as a working margin, with the archive on external volumes that are catalogued and stored offline.
What tools are worth using, and what do they cost in time?
Disk analyzers show where space went; they do not decide what may go. A visual map of a 1 TB drive takes a few minutes to build and is useful once per project, not once per day. Terminal commands `du` and `df` are faster for a single question and require no installation.
Paid cleaning applications vary in what they remove and in how they report it. The question to ask is whether the tool shows the full path of every item before deletion and whether it can exclude a folder. A tool that deletes without a preview is a risk on a drive that holds the only copy of a frame set.
For a capture day, the reliable sequence is short: measure with `df -h /` and the Storage pane, verify that the external archive is complete, remove duplicate offloads and caches, thin local snapshots if the gap is large, and leave the originals alone. The measurement takes two minutes. Recovering a deleted frame set does not.
How much margin should a capture day keep?
Plan for the raw set, the proxies, the project file, the application cache and the operating system's own writes. A working figure is 1.5 times the expected raw volume as free space at the start of the day, with 100 GB as a floor for a machine that edits on the internal drive. If the cards total 128 GB and the proxies add 40 GB, the drive should show at least 250 GB free before the first offload.
If the number is short, the decision is made in this order: move an archived project to external storage, delete proxies for delivered work, clear caches, thin snapshots. Deleting originals is not part of the sequence. A drive that is measured, verified and trimmed the evening before will hold the day's frames; a drive that is guessed at will not. The same question is worked through in PC build and repair guides.