What is the SFCACHE file format?
A .sfcache file is a cache file created by Microsoft's Windows ReadyBoost technology, first introduced in Windows Vista and supported through Windows 10 and 11. The .sfcache file is compatible with USB devices such as USB flash drives, SD cards, and memory cards, and is always stored at the root of the removable drive under the fixed name ReadyBoost.sfcache.
ReadyBoost uses the flash storage to cache frequently accessed disk data and improve system performance - not to add virtual RAM, as is sometimes stated. Windows encrypts the cached content with AES-128 before writing it to the device, so data remains protected even if the drive is lost or shared.
The file is managed entirely by Windows through the SuperFetch service (called SysMain in Windows 10 and 11): ReadyBoost.sfcache is created automatically when ReadyBoost is enabled on a device, and deleted when the feature is turned off or the drive is safely ejected. Users cannot open or edit it manually.
Security & safety
RISK: LOWReadyBoost.sfcache is a legitimate Windows system file and is safe; it holds only encrypted cache data, so deleting it cannot lose personal data (Windows recreates or omits it). The only caveats: don't yank the USB drive while ReadyBoost is active (disable it first to avoid cache-write errors), and be wary of any "sfcache repair/recovery" tool you find online - there is nothing to repair and such utilities are often PUP/adware. Malware occasionally mimics system file names, so verify the file is the genuine ReadyBoost.sfcache in a drive's root if anything seems off.
Format details
in a nutshellPrograms that open SFCACHE files
Windows1 app
Microsoft Windows (ReadyBoost)
Built-in There is nothing to open. Manage it via the drive's Properties > ReadyBoost tab; to remove it, choose 'Do not use this device' and Windows deletes the file. Technical details
deep spec| Developer | Microsoft Corporation |
| File name on device | Always `ReadyBoost.sfcache` - the name is fixed and assigned by Windows |
| Storage location | Root directory of the ReadyBoost-enabled removable drive |
| Encryption | AES-128; the entire payload is encrypted by Windows before being written to the device |
| Compression | Cache data is also compressed to maximise effective cache capacity on the flash device |
| Cache size per device | 250 MB to 32 GB (Windows 7 and later); capped at 4 GB on Windows Vista |
| Device speed requirement | At least 2.5 MB/s for 4 KB random reads and 1.75 MB/s for 512 KB random writes |
| Underlying Windows service | SuperFetch (service host name SysMain in Windows 10 and 11) |
| File identification method | Identified by file name and path only; no public magic bytes or documented internal header |
| Multi-device support | Up to 8 simultaneous ReadyBoost-enabled devices (Windows 7 and later) |
| Format type | OS-managed encrypted binary cache; internal layout is undocumented and proprietary |
| MIME type | `application/octet-stream` |
| Supported OS versions | Windows Vista, 7, 8, 8.1, 10, 11 |
| File lifecycle | Created automatically when ReadyBoost is enabled on a device; deleted when the feature is disabled or the drive is ejected |
| Released | Windows Vista (2007), with the ReadyBoost feature |
| Specification | learn.microsoft.com |
SFCACHE conversions
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