A nearly full C: drive can block Windows updates and new applications, but the capacity shown in File Explorer is only the start of the diagnosis. First check which folders and volumes use space, then choose cleanup, relocation, or a hardware upgrade.
Step 1. Open File Explorer > This PC. Under Devices and drives, read the used/free bar for C:, D:, and any external drives. If a drive is missing, open Disk Management before assuming it is empty.
Step 2. Right-click a volume and open Properties to see used and free bytes. The total shown for C: is the size of that partition, not necessarily the full physical disk. A 1 TB disk can be split into C:, D:, EFI, and recovery partitions.
Step 3. Open Disk Management and note disk numbers, partition sizes, and any unallocated space. Do not delete a small unlabeled EFI or recovery partition to gain a little space; it may be required for boot or reset.
Open Settings > System > Storage. Wait for Windows to calculate categories such as Apps, Temporary files, Documents, and System & reserved. For another volume, open View storage usage on other drives or its equivalent in your Windows 10 build. The Storage page is an estimate and can differ from the volume's raw free-space figure because of reserved storage and files owned by other accounts.
Check large applications in Apps & features and game libraries in their launchers. Some hidden data is in user profiles, caches, virtual machines, or restore points; do not remove folders manually unless you know their purpose. If an external disk holds a backup, do not count its capacity as free space on C:.
Step 1. Back up irreplaceable documents to a separate disk and open sample files from the backup.
Step 2. In Storage, open Temporary files or Cleanup recommendations. Review every selected category. Leave Downloads and previous Windows installation files intact if you still need them or a rollback path.
Step 3. Remove only confirmed disposable files. Uninstall unused programs through Windows Settings and move supported games through their launcher rather than dragging their folders.
Step 4. Recheck C: in File Explorer. Leave working headroom for updates, temporary files, and future installs; the exact amount depends on workload and disk size.
If unallocated space is directly next to C: on the same physical disk, an extension may be possible after a full backup. If another partition lies between them, moving partitions adds risk and requires a clear recovery plan. Space on a different disk cannot be added to C: by simply changing a drive letter. A physically full disk needs files moved elsewhere or a larger replacement.
Qiling Disk Master can help inspect and resize partitions, but do not start a resize until you have identified the exact disk and made a verified backup. A resize in progress should not be interrupted merely because it appears slow.
Measure free space in File Explorer, identify the cause in Storage and Disk Management, then clean, relocate, resize, or upgrade only after protecting the files you need.