Low disk space: what to delete and what to leave alone

Low disk space is one of the most frequent alerts monitoring raises, and one of the few where a well-meaning user can make things worse in about thirty seconds. This is the article I would rather send you than talk you through it on the phone while you are hovering over a folder you should not delete.

Why it matters

Windows needs free space to work. Once the C: drive drops below roughly 10 percent free, you start to see:

• Updates failing to install, which means missed security patches.
• Applications crashing or refusing to save.
• Outlook unable to sync, which looks like an email problem but is not.
• The whole machine slowing down, because Windows uses disk space as overflow memory.

Aim for at least 15 to 20 percent free at all times.

Safe to clear

• Downloads folder. Almost always the biggest single win. Installers, PDFs and files you downloaded once, two years ago.
• Recycle Bin. Deleting a file does not free the space until the bin is emptied.
• Windows Disk Cleanup, including "Clean up system files". Removes temporary files, old update files and previous Windows installations. This can recover 20GB or more after a feature update.
• Old Teams and browser caches. They rebuild themselves automatically.
• Duplicated files sitting in both OneDrive and a local folder.

Leave these alone

• Anything in C:\Windows. No exceptions.
• Program Files folders. Uninstall applications properly through Settings, do not delete the folder.
• Files with names you do not recognise in the root of C:.
• OneDrive or SharePoint folders, unless you are certain the file is synced. Deleting locally deletes it in the cloud for everyone.
• "Old" user profile folders. These often still hold someone's desktop and documents.

The one that catches people out

OneDrive Files On-Demand shows cloud files in Explorer as if they are local. If you have ever right-clicked a folder and chosen "Always keep on this device", you have downloaded the entire thing onto a drive that may not have room for it. Setting large, rarely-used folders back to "Free up space" is often the cleanest fix, and nothing is lost.

Stopping it happening again

• Keep working files in OneDrive or SharePoint rather than the desktop.
• Empty Downloads monthly. Make it part of the same routine as your weekly restart.
• If a machine fills up repeatedly despite all this, the drive is simply too small for the job and it is cheaper to replace it than to keep firefighting.

What I do at my end

When a disk space alert comes in on a monitored device, I clear the recoverable space, confirm the free capacity is healthy and note it on your report. If it keeps recurring on the same machine, I will flag it as a hardware conversation rather than clearing it silently every month.

Not sure what is safe to remove on your machine? Get in touch before you delete anything.