What is the DT file format?
Earlier versions of Apple's classic Macintosh stored files using a dual-fork structure: a resource fork and a data fork. The .dt extension was an informal convention used by early cross-platform file-transfer utilities to preserve the data fork as a separate sidecar file when moving a Mac file to a non-HFS volume.
A fork file has no fixed size limits - it mirrors the data fork of the original file exactly, and at times could exceed the resource fork in size. The .dt file carries no special header of its own; its bytes are the raw data fork contents, while the resource fork was stored in a separate companion file. Macintosh Data Fork Files were used on earlier Mac versions and could not be accessed by the general user - they were more specifically present for system-level file transfer.
Modern macOS handles forks transparently via extended attributes, and cross-platform transfers now produce AppleDouble __MACOSX companion files inside ZIP archives instead. A .dt file encountered today is almost certainly a leftover from a pre-OS X migration or a classic Mac emulator such as BasiliskII.
Security & safety
RISK: LOWRaw data fork files pose no inherent security risk - they are passive data files. In the context of classic Mac emulation, the usual caution around software from untrusted sources applies.
Format details
in a nutshell- Desktop database file (Mac OS System folder) - Mac OS 7-9 stored the desktop database as 'Desktop DB' and 'Desktop DF' - sometimes referenced by tools as .dt files; these were invisible system files managing the Finder's icon and document-type records.
- Various application data files - Some Windows and Linux applications use .dt as a generic data file extension (e.g. some game engines, scientific software); these are entirely unrelated to Mac forks.
Programs that open DT files
Windows1 app
HFVExplorer
Free Explore HFS disk images from classic Mac emulators; can handle data/resource fork split files. 
macOS2 apps
Mac OS X / macOS (built-in)
Built-in Try opening the .dt file directly with the application that created the original document. The data fork often contains the readable content (text, image data). If the original app is not available, try a hex editor.
BasiliskII (classic Mac emulator)
Open-source Run a classic Mac OS system image in BasiliskII; use HFVExplorer or Disk Copy to import .dt files back onto an HFS disk image for use with period-correct apps. Technical details
deep spec| Magic bytes | No fixed magic; content mirrors the data fork of the original file, which varies by type |
| Byte order | Depends on original file type; typically big-endian for classic Mac data structures |
| Encoding | Binary or text, matching the original file's data fork content exactly |
| Container | None - raw data fork bytes with no added header or wrapper of any kind |
| Data storage | Bytes are identical to the original data fork - a `.dt` from a PICT image contains raw PICT data; from a text file, raw text |
| HFS fork context | Classic HFS supported exactly two forks per file; `.dt` isolates the data fork while the resource fork is stored separately |
| Typical size | Variable; identical to the source file's data fork (bytes to several MB) |
| Companion file | Resource fork stored separately, typically as a `.rsrc` or AppleDouble sidecar |
| Associated OS | Classic Mac OS (System 1 - Mac OS 9); early Mac OS X transition period |
| Transfer context | Created by cross-platform file-transfer utilities splitting HFS dual-fork files for non-HFS volumes |
| Modern equivalent | AppleDouble `__MACOSX/` folder (in ZIP transfers); APFS extended attributes on macOS |
| Filesystem origin | HFS (Hierarchical File System), introduced with Mac OS in 1985 |
| Released | ~1985 (HFS / classic Mac OS, though .dt suffix usage is informal) |
DT conversions
Community Q&A
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