Formatting a drive wipes all the data off it, giving you a clean slate to start afresh from. It also allows you to change the file system of the drive. You might need to alter this to use the drive with a different kind of device or if an app doesn’t support the default file system. If you've been affected by OneDrive's recent refusal to sync to non-NTFS drives, you can follow this guide to reformat an SD card to use NTFS.
Formatting a drive is a simple procedure. As a final reminder before you get started, formatting will permanently delete everything you have stored on the drive. You won't be able to recover your files later unless you've made a backup first.
To start a format, right-click the drive you want to wipe in File Explorer and click the "Format" option. You can also open the Format window from the "Drive Tools" tab in File Explorer's Ribbon if you've already selected a drive.
The Format window lets you initiate a format and change the drive's settings. You can specify the file system to format the drive with under the "File system" dropdown menu. The supported file systems for your drive will be listed. NTFS is the default for most internal hard drives while variations of FAT are generally used for memory cards and USB sticks. If you want to restore your device's default file system, you can press the "Restore device defaults" button.
You shouldn't need to change the "Allocation unit size" option. This is used by some file systems to set the block size of the drive. A "block" is the minimum amount of drive space any file can occupy. If you have a lot of small files, reducing the allocation unit size will lower the amount of space required. Keeping the block size higher could improve performance with larger files by reducing the number of blocks that need to be searched during seek operations. Generally, the default option will be the best for general use.
When you're ready to format your drive, press the "Start" button and acknowledge the final warning. All your data will be deleted and the file system you selected will be applied. If you leave the "Quick Format" checkbox ticked, the process should only take a few seconds. Sometimes you may need to complete a full format by disabling the Quick Format option. This takes considerably longer but also scans the drive to identify any bad sectors. You should use this option if you're reformatting a drive you suspect may be defective.