diff --git a/src/LibHac/FsSystem/UnionStorage.cs b/src/LibHac/FsSystem/UnionStorage.cs
new file mode 100644
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+++ b/src/LibHac/FsSystem/UnionStorage.cs
@@ -0,0 +1,352 @@
+using System;
+using LibHac.Common;
+using LibHac.Common.FixedArrays;
+using LibHac.Diag;
+using LibHac.Fs;
+using LibHac.Os;
+using LibHac.Util;
+
+namespace LibHac.FsSystem;
+
+/// <summary>
+/// An <see cref="IStorage"/> that allows modifying a base storage by writing any modifications to a separate
+/// "log" storage.
+/// </summary>
+/// <remarks><para>The storage is split into equally-sized blocks. When a block is modified for the first time,
+/// a new entry containing the block's data will be added to the log. Any subsequent read/write operations
+/// on that block will use the log storage entry instead of the base storage.</para>
+/// <para>Log format:
+/// <code>
+/// s64 BlockSize;
+/// Entry Entries[];
+///
+/// Entry {
+///    s64 BaseStorageOffset;
+///    u8 Data[BlockSize];
+/// };
+/// </code></para>
+/// <para>Based on FS 13.1.0 (nnSdk 13.4.0)</para></remarks>
+public class UnionStorage : IStorage
+{
+    private const long Sentinel = -1;
+    private const int LogHeaderSize = 8;
+    private const int LogEntryHeaderSize = 8;
+
+    private ValueSubStorage _baseStorage;
+    private ValueSubStorage _logStorage;
+    private long _blockSize;
+    private byte[] _buffer;
+    private int _blockCount;
+    private SdkMutexType _mutex;
+
+    public UnionStorage()
+    {
+        _mutex = new SdkMutexType();
+    }
+
+    public override void Dispose()
+    {
+        _baseStorage.Dispose();
+        _logStorage.Dispose();
+        _buffer = null;
+
+        base.Dispose();
+    }
+
+    private static long GetLogSize(long blockSize)
+    {
+        return blockSize + LogEntryHeaderSize;
+    }
+
+    private static long GetDataOffset(long logOffset)
+    {
+        return logOffset + LogEntryHeaderSize;
+    }
+
+    private static long GetLogTailOffset(long blockSize, int blockCount)
+    {
+        return GetLogSize(blockSize) * blockCount + LogHeaderSize;
+    }
+
+    public static Result Format(in ValueSubStorage storage, long blockSize)
+    {
+        Assert.SdkRequiresGreater(blockSize, 1);
+        Assert.SdkRequires(BitUtil.IsPowerOfTwo(blockSize));
+
+        var header = new Array2<long>();
+        header[0] = blockSize;
+        header[1] = Sentinel;
+
+        return storage.Write(0, SpanHelpers.AsReadOnlyByteSpan(in header));
+    }
+
+    public Result Initialize(in ValueSubStorage baseStorage, in ValueSubStorage logStorage, long blockSize)
+    {
+        Assert.SdkRequiresNull(_buffer);
+
+        Result rc = logStorage.Read(0, SpanHelpers.AsByteSpan(ref _blockSize));
+        if (rc.IsFailure()) return rc.Miss();
+
+        if (blockSize <= 1 || !BitUtil.IsPowerOfTwo(blockSize) || blockSize != _blockSize)
+            return ResultFs.InvalidLogBlockSize.Log();
+
+        // Read through the log to see if we already have any existing entries
+        for (long offset = LogHeaderSize; ; offset += GetLogSize(_blockSize))
+        {
+            long offsetOriginal = 0;
+            rc = logStorage.Read(offset, SpanHelpers.AsByteSpan(ref offsetOriginal));
+            if (rc.IsFailure()) return rc.Miss();
+
+            if (offsetOriginal == Sentinel)
+                break;
+
+            if (offsetOriginal % _blockCount != 0)
+                return ResultFs.InvalidLogOffset.Log();
+
+            _blockCount++;
+        }
+
+        _baseStorage.Set(in baseStorage);
+        _logStorage.Set(in logStorage);
+        _buffer = new byte[_blockSize];
+
+        return Result.Success;
+    }
+
+    public Result Freeze()
+    {
+        Assert.SdkRequiresNotNull(_buffer);
+
+        using ScopedLock<SdkMutexType> scopedLock = ScopedLock.Lock(ref _mutex);
+
+        long tailOffset = GetLogTailOffset(_blockSize, _blockCount);
+        long value = Sentinel;
+        Result rc = _logStorage.Write(tailOffset, SpanHelpers.AsReadOnlyByteSpan(in value));
+        if (rc.IsFailure()) return rc.Miss();
+
+        rc = _logStorage.Flush();
+        if (rc.IsFailure()) return rc.Miss();
+
+        return Result.Success;
+    }
+
+    public Result Commit()
+    {
+        Assert.SdkRequiresNotNull(_buffer);
+
+        using ScopedLock<SdkMutexType> scopedLock = ScopedLock.Lock(ref _mutex);
+
+        long tailOffset = GetLogTailOffset(_blockSize, _blockCount);
+        long logSize = GetLogSize(_blockSize);
+
+        // Read each block of data from the log storage and write it to the base storage
+        for (long offset = LogHeaderSize; offset < tailOffset; offset += logSize)
+        {
+            long offsetOriginal = 0;
+            Result rc = _logStorage.Read(offset, SpanHelpers.AsByteSpan(ref offsetOriginal));
+            if (rc.IsFailure()) return rc.Miss();
+
+            if (offsetOriginal == Sentinel)
+                return ResultFs.UnexpectedEndOfLog.Log();
+
+            rc = _logStorage.Read(GetDataOffset(offset), _buffer);
+            if (rc.IsFailure()) return rc.Miss();
+
+            rc = _baseStorage.Write(offsetOriginal, _buffer);
+            if (rc.IsFailure()) return rc.Miss();
+        }
+
+        return _baseStorage.Flush();
+    }
+
+    public override Result Read(long offset, Span<byte> destination)
+    {
+        Assert.SdkRequiresNotNull(_buffer);
+
+        if (destination.IsEmpty)
+            return Result.Success;
+
+        // Get the start offset of the block containing the requested offset
+        long offsetBuffer = 0;
+        long offsetOriginal = Alignment.AlignDownPow2(offset, _blockSize);
+        long sizeSkipBlock = offset - offsetOriginal;
+
+        while (offsetBuffer < destination.Length)
+        {
+            // Determine how much of the block we should read
+            long sizeReadBlock = _blockSize - sizeSkipBlock;
+            long sizeRemaining = destination.Length - offsetBuffer;
+            long sizeToRead = Math.Min(sizeReadBlock, sizeRemaining);
+            Span<byte> currentDestination = destination.Slice((int)offsetBuffer, (int)sizeToRead);
+
+            // Check if the log contains the block we need
+            Result rc = FindLog(out bool found, out long offsetLog, offsetOriginal);
+            if (rc.IsFailure()) return rc.Miss();
+
+            // If it does, read from the log; otherwise read from the base storage
+            if (found)
+            {
+                rc = _logStorage.Read(GetDataOffset(offsetLog) + sizeSkipBlock, currentDestination);
+                if (rc.IsFailure()) return rc.Miss();
+            }
+            else
+            {
+                rc = _baseStorage.Read(offsetOriginal + sizeSkipBlock, currentDestination);
+                if (rc.IsFailure()) return rc.Miss();
+            }
+
+            offsetBuffer += sizeToRead;
+            offsetOriginal += _blockSize;
+            sizeSkipBlock = 0;
+        }
+
+        return Result.Success;
+    }
+
+    public override Result Write(long offset, ReadOnlySpan<byte> source)
+    {
+        Assert.SdkRequiresNotNull(_buffer);
+
+        if (source.IsEmpty)
+            return Result.Success;
+
+        using ScopedLock<SdkMutexType> scopedLock = ScopedLock.Lock(ref _mutex);
+
+        // Get the start offset of the block containing the requested offset
+        long offsetBuffer = 0;
+        long offsetOriginal = Alignment.AlignDownPow2(offset, _blockSize);
+        long sizeSkipBlock = offset - offsetOriginal;
+
+        while (offsetBuffer < source.Length)
+        {
+            Assert.SdkNotEqual(Sentinel, offsetOriginal);
+
+            long sizeWriteBlock = _blockSize - sizeSkipBlock;
+            long sizeRemaining = source.Length - offsetBuffer;
+            long sizeToWrite = Math.Min(sizeWriteBlock, sizeRemaining);
+            ReadOnlySpan<byte> currentSource = source.Slice((int)offsetBuffer, (int)sizeToWrite);
+
+            // Check if the log contains the block we need.
+            Result rc = FindLog(out bool found, out long offsetLog, offsetOriginal);
+            if (rc.IsFailure()) return rc.Miss();
+
+            if (found)
+            {
+                // If it does, write directly to the log.
+                _logStorage.Write(GetDataOffset(offsetLog) + sizeSkipBlock, currentSource);
+                if (rc.IsFailure()) return rc.Miss();
+            }
+            else
+            {
+                // Otherwise we need to add a new entry to the log.
+                _logStorage.Write(offsetLog, SpanHelpers.AsReadOnlyByteSpan(in offsetOriginal));
+                if (rc.IsFailure()) return rc.Miss();
+
+                if (sizeToWrite == _blockSize)
+                {
+                    // If we're writing a complete block we can write the entire block directly to the log.
+                    _logStorage.Write(GetDataOffset(offsetLog) + sizeSkipBlock, currentSource);
+                    if (rc.IsFailure()) return rc.Miss();
+                }
+                else
+                {
+                    // If we're writing a partial block we need to read the existing data block from the base storage
+                    // into a buffer first.
+                    _baseStorage.Read(offsetOriginal, _buffer);
+                    if (rc.IsFailure()) return rc.Miss();
+
+                    // Fill in the appropriate parts of the buffer with our new data.
+                    currentSource.CopyTo(_buffer.AsSpan((int)sizeSkipBlock, (int)sizeToWrite));
+
+                    // Write the entire modified block to the new log entry.
+                    _logStorage.Write(GetDataOffset(offsetLog), _buffer);
+                    if (rc.IsFailure()) return rc.Miss();
+                }
+
+                _blockCount++;
+            }
+
+            offsetBuffer += sizeToWrite;
+            offsetOriginal += _blockSize;
+            sizeSkipBlock = 0;
+        }
+
+        return Result.Success;
+    }
+
+    public override Result Flush()
+    {
+        Assert.SdkRequiresNotNull(_buffer);
+
+        Result rc = _baseStorage.Flush();
+        if (rc.IsFailure()) return rc.Miss();
+
+        rc = _logStorage.Flush();
+        if (rc.IsFailure()) return rc.Miss();
+
+        return Result.Success;
+    }
+
+    public override Result GetSize(out long size)
+    {
+        Assert.SdkRequiresNotNull(_buffer);
+
+        return _baseStorage.GetSize(out size);
+    }
+
+    public override Result SetSize(long size)
+    {
+        return ResultFs.UnsupportedSetSizeForUnionStorage.Log();
+    }
+
+    public override Result OperateRange(Span<byte> outBuffer, OperationId operationId, long offset, long size,
+        ReadOnlySpan<byte> inBuffer)
+    {
+        for (long currentOffset = Alignment.AlignDownPow2(offset, _blockSize);
+             currentOffset < offset + size;
+             currentOffset += _blockSize)
+        {
+            Result rc = FindLog(out bool found, out long offsetLog, currentOffset);
+            if (rc.IsFailure()) return rc.Miss();
+
+            if (found)
+            {
+                _logStorage.OperateRange(outBuffer, operationId, offsetLog, _blockSize, inBuffer);
+                if (rc.IsFailure()) return rc.Miss();
+            }
+        }
+
+        return _baseStorage.OperateRange(outBuffer, operationId, offset, size, inBuffer);
+    }
+
+    private Result FindLog(out bool logFound, out long outLogOffset, long offsetOriginal)
+    {
+        Assert.SdkRequiresNotNull(_buffer);
+
+        outLogOffset = GetLogTailOffset(_blockSize, _blockCount);
+        logFound = false;
+
+        long logTailOffset = GetLogTailOffset(_blockSize, _blockCount);
+        long logSize = GetLogSize(_blockSize);
+
+        // Go through each log entry to see if any are at the requested offset
+        for (long logOffset = LogHeaderSize; logOffset < logTailOffset; logOffset += logSize)
+        {
+            long offset = 0;
+            Result rc = _logStorage.Read(logOffset, SpanHelpers.AsByteSpan(ref offset));
+            if (rc.IsFailure()) return rc.Miss();
+
+            if (offset == Sentinel)
+                return ResultFs.LogNotFound.Log();
+
+            if (offset == offsetOriginal)
+            {
+                outLogOffset = logOffset;
+                logFound = true;
+                break;
+            }
+        }
+
+        return Result.Success;
+    }
+}
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