What is the RSF file format?
An .rsf file is a binary sidecar file created by OWC MacDrive (formerly Mediafour MacDrive) when a Mac file that carries a resource fork is copied to a Windows filesystem such as NTFS or FAT32. Classic Mac OS used a dual-fork file system: every file could have a data fork (the main content) and a resource fork (application-specific metadata, icons, code, or custom data). Windows filesystems cannot natively store this two-fork structure, so MacDrive splits them - the data fork stays in the main file, and the resource fork is preserved in a companion .rsf sidecar sharing the same base name.
The internal structure mirrors the classic Mac OS resource fork layout: a resource map and resource data section, stored in big-endian binary. Because resource forks are a legacy feature of Mac OS 9 and earlier, most files produced by modern macOS carry no resource fork at all, making .rsf sidecars rare in current workflows. Deleting an .rsf file without removing the paired main file can cause the associated Mac application to lose metadata or custom attributes stored in its resource fork.
MacDrive was rebranded as OWC MacDrive in the 2020s; version 11.1.4.26 was released in February 2026 and remains the only software capable of reading or creating .rsf files.
Security & safety
RISK: LOWRSF is a binary companion data file - no executable code, no scripts. Safe to encounter; do not delete if you need the original Mac file's resource fork data. Download MacDrive only from the official mediafour.com or OWC site to avoid unofficial repackages.
Format details
in a nutshell- Remote Support Force Script - Some remote-support and automation tools use .rsf as a proprietary script or settings file - unrelated to MacDrive.
Programs that open RSF files
Windows1 app
MacDrive (OWC MacDrive)
Paid MacDrive handles .rsf files automatically when reading Mac-formatted drives. No manual open action needed; MacDrive reunites the .rsf with its companion file transparently. Technical details
deep spec| File type | Binary sidecar - stores the Macintosh resource fork of the corresponding main file |
| Byte order | Big-endian (mirrors classic Mac OS resource fork structure) |
| Paired file | Shares the base name with its data-fork partner; e.g., MyDoc.doc + MyDoc.rsf |
| Typical file size | Under 1 KB to several MB, depending on the original resource fork content |
| Internal structure | Resource map (index of resource types and IDs) + resource data section - mirrors classic Mac OS HFS/HFS+ resource fork layout |
| Data storage | Flat binary; resource data stored as-is without any compression layer |
| Target platform | Windows (NTFS or FAT32 volumes managed by MacDrive) |
| Creator software | OWC MacDrive (formerly Mediafour MacDrive); version 11.1.4.26 as of February 2026 |
| Format origin | Classic Mac OS HFS/HFS+ dual-fork file system; resource fork concept dates to Mac OS 1 (1984) |
| Modern prevalence | Rare - most contemporary macOS files carry an empty resource fork, so .rsf sidecars seldom appear |
| Deletion risk | Deleting the .rsf without removing the parent file can cause the associated Mac app to lose fork-stored metadata |
| Proprietary format | No public specification; only OWC MacDrive reads and writes .rsf files |
| Released | early 2000s (MacDrive 5.x era) |
| Latest version | MacDrive 11.1.4.26 (February 2026; now OWC MacDrive) |
| Specification | www.mediafour.com |
RSF conversions
Community Q&A
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