Summary

Fix failures when writing Windows disk images stored as compressed fixed VHDs such as .vhd.zst.

Problem

When writing a compressed Windows image to a CFast card, Rufus could fail shortly after pressing START with a compressed-image write error.

The immediate write failure was caused by sector-sized writes reaching the physical device from decompressor buffers that were not sector-aligned. Some physical media accept sector-sized writes only when the source buffer is aligned as well, so the write path could fail even though the decompression itself was fine.

There was also a second issue specific to compressed fixed VHD payloads. Rufus could treat a .vhd.zst image like a generic compressed raw image and fall back to the compressed archive size instead of the decompressed disk payload size. That could cause the write path to stop at the archive size instead of the real end of the fixed VHD payload.

Fix

This change:

• routes unaligned full-sector decompressor output through an aligned bounce buffer before writing to the target device
• keeps the existing partial-sector buffering behavior for compressed-image writes
• derives the decompressed content size for zstd-backed fixed VHD payloads and uses that payload size when deciding where the real disk data ends
• avoids truncating compressed fixed-VHD writes to the compressed archive size

Result

Compressed .vhd.zst images now write correctly to physical media that require aligned raw writes, and fixed-VHD payloads are written to their proper payload length instead of being cut off at the compressed archive size.