Tool
Read an exFAT boot sector
Is the image really exFAT? And how is it laid out? The answer sits in the first sectors.
The short answer
The boot sector describes the layout of the whole device.
The volume boot record of an exFAT device holds everything needed to evaluate it: sector and cluster size, position and length of the FAT, the start of the cluster heap and the first cluster of the root directory. This tool reads it locally. Everything runs locally in your browser. There is no upload, no server request and no storage.
01 / Read
Choose an image or device file
02 / Understand
What the values mean
If the tool finds no exFAT boot sector at the start, it additionally looks for a partition in the MBR partition table and reads its beginning. Images of a single partition and images of a whole device therefore both work.
- Sector size: the smallest addressable unit, usually 512 bytes, on some media 4096 bytes.
- Cluster size: the smallest allocation unit for files. Large clusters reduce management overhead but waste space on small files.
- FAT offset and FAT length: where the file allocation table starts and how long it is. It chains the clusters of fragmented files.
- Cluster heap offset: the actual file data begins here.
- Root directory cluster: the entry point into the directory structure.
- In use per header: a coarse percentage maintained by the file system. It is an indication, not an exact occupancy figure.
Sources and technical references
Common questions
Is my image uploaded?
No. Only the first two megabytes are read, and exclusively inside the browser. A 128 gigabyte image is therefore evaluated just as quickly as a small file.
Why does the tool find no exFAT?
It may be a different file system, a compressed image, a GPT device with an unusual layout, or a single file rather than a disk image. The hex dump shown helps with the classification.
Can I check my memory card directly?
For security reasons browsers cannot access a raw device directly. Create an uncompressed image of the card first and drop that here.