Why Files Disappear from Memory Cards: Understanding exFAT Table Wear
- scarlettkim7
- Jun 29
- 5 min read
What Is a File System?
A file system is the method used by an operating system (OS) in a computer or camera to organize and manage data on a storage device.
An important point is that a file system treats all storage media the same, regardless ofwhether the underlying device is:
an HDD
an SSD
a memory card
or an optical disc
All of them are managed using the same logical structure, independent of the physical storage technology.
Why 512 Bytes per Sector?
For historical reasons, file systems divide storage devices into 512-byte units called
sectors.
This 512-byte size originates from:
the earliest hard disk drives, and
loppy disks, which are rarely used today
The value has been preserved for compatibility and legacy reasons and remains in use
today.
In other words:
512-bytes is a purely logical unit and has no direct relationship to the physical
structure of NAND flash memory.
Clusters: The Actual Management Unit of a File System
From the operating system’s perspective, a 512-byte sector is too small to manage
efficiently.
Therefore, file systems group multiple sectors together into a larger unit called
a cluster.
In the exFAT file system, which is widely used in cameras:
the cluster size depends on the card capacity, but
it is typically 128 KB to 512 KB.
Structure of a 64 GB SD Card Formatted with exFAT
A 64 GB SD card formatted with exFAT is roughly divided as follows:
About 32 MB (0.05%) → File system area (boot information and tables that store
file and folder locations)
About 99.95% → Data area where actual video files are stored
The file system manages the data area in cluster units.
For historical reasons, when N clusters are available:
the first usable cluster is numbered Cluster 2
the last cluster is numbered Cluster N+1, where N is the total number of clusters
Why a 1-Byte File Uses 128 KB
For example:
On an SD card with a 128 KB cluster size
Creating a file that is only 1 byte in size
Still results in:
one full 128 KB cluster being allocated
This is why, in Windows file properties, you see:
“File size”
and “Size on disk”
showing different values. The “size on disk” is always a multiple of the cluster size.
Why Not Manage Data in Sector Units?
This is a choice made for management efficiency.
For a 64 GB SD card:
Managing data in 512-byte sectors would require tracking about 128 million sectors
By contrast:
Managing data in 128 KB clusters requires tracking
only about 640,000 clusters
From a file system’s perspective, cluster-based management is far more practical and
efficient.
Why the File System Area Wears Out Faster
During recording, two fundamentally different types of data are written to the memorycard:
1. Video data – the actual recorded footage
2. File system data – information that manages where and how the video data is
stored
These two data types behave very differently.
Data Write Characteristics During Recording
What Actually Happens During Recording?
When recording video:
1. Video data
is written sequentially
into the data area
typically only once
2. File system tables, however:
are updated continuously
as new clusters are allocated
and as file size and location information changes
In short:
Video data is written once, but file system tables are rewritten continuously throughoutrecording.
Why the Table Area Wears Out First
The file system tables contain:
lists of clusters used by a file
pointers to the next cluster
file size growth information
folder and directory structures
Although each update may involve only a few bytes, these updates:
occur repeatedly during recording
target the same logical addresses
As a result:
the same logical addresses (LBAs) are repeatedly rewritten
data is constantly relocated to new physical pages (PBAs)
garbage collection and folding activity increases
This is why:
The file system area is almost always the first region of a memory card to accumulate
fatigue.
The Dangerous Part: “The Video Looks Fine”
If the table area becomes corrupted:
the video data may still physically exist
but files and folders may not appear
or the memory card may not be recognized at all
This leads to common real-world scenarios such as:
“The recording finished normally, but the files are gone”
“The card looks empty when inserted”
“The camera or PC says the card is corrupted”
Why exFAT Is Especially Vulnerable
The exFAT file system:
does not use journaling
so if an error occurs during a table update
recovery options are very limited
Power loss, internal latency, or garbage collection conflicts often damage the table
area first for this reason.
How OFFLOADER Avoids This exFAT Weakness
OFFLOADER is designed to fundamentally avoid the structural weaknesses of exFAT.
In conventional backup workflows:
file and folder creation happens repeatedly during copying
file system tables are constantly accessed and updated
causing concentrated wear and increased GC activity
OFFLOADER uses a two-stage backup process:
Stage 1
All required folders and files are created in advance
Storage space is allocated in a single operation
A TMP folder is created in the data area
Stage 2
During backup, no further file system table access occurs
Only raw data is copied sequentially into the data area
Once the backup completes successfully:
the TMP folder is simply renamed to the final backup folder
This approach:
reduces file system access to a single operation
minimizes card fatigue
and effectively eliminates unnecessary garbage collection during backup
A Folder Is Also “Just Data”
One interesting point worth noting:
Files contain recorded video data
Folders are special data files that store names, timestamps, and location
information
In other words:
a folder also occupies a cluster in the data area
it is simply another type of file
The figure below shows the root directory data of a memory card formatted by a Sony camera.
Among the numerical data, the folder name [AVF_INFO] can be seen embedded in
plain text.
This demonstrates that:
A folder is not just a concept—it is a real data structure stored on the memory card.
So What Problems Can Occur?
If video file data is corrupted → playback may fail or stop midway
If folder or table data is corrupted → files may disappear from the file browser
entirely
For a deeper understanding of why memory card data can become corrupted, and why recording may stop even when a card appears normal, please refer to the related articles below.
1. Why a Memory Card That Looks Fine Can Ruin Your Shoot
2. Why Recording Stops During Long Continuous Shoots (7 Common Causes)
3. Why Memory Cards Become Unstable: Over-Provisioning, FTL, and Garbage Collection
4. Why V30 / V90 Memory Cards Can Still Fail During Recording
Comments