diff --git a/firmware/include/fs_defines.h b/firmware/include/fs_defines.h
index 70e083ae2e..f4a9fa2d3e 100644
--- a/firmware/include/fs_defines.h
+++ b/firmware/include/fs_defines.h
@@ -107,11 +107,31 @@
 #define SECTOR_SIZE     512
 #endif
 
-/* this _could_ be larger than a sector if that would ever be useful */
-#ifdef MAX_VARIABLE_LOG_SECTOR
-#define DC_CACHE_BUFSIZE   MAX_VARIABLE_LOG_SECTOR
-#else
-#define DC_CACHE_BUFSIZE    SECTOR_SIZE
+/* The file I/O code operates in terms of the FILESYSTEM sector size,
+   which may be much larger than the underlying device's logical sector size.
+   So we have to use the larger of the filesystem's max, the max logical, or
+   the the fixed logical size.
+*/
+
+#if defined(MAX_VIRT_SECTOR_SIZE) && defined(MAX_VARIABLE_LOG_SECTOR)
+#if (MAX_VIRT_SECTOR_SIZE < MAX_VARIABLE_LOG_SECTOR)
+#error "MAX_VIRT_SECTOR_SIZE < MAX_VARIABLE_LOG_SECTOR"
+#endif
+#if (MAX_VIRT_SECTOR_SIZE % MAX_VARIABLE_LOG_SECTOR)
+#error "MAX_VIRT_SECTOR_SIZE is not a multiple of MAX_VARIABLE_LOG_SECTOR"
+#endif
+#endif
+
+#if !defined(DC_CACHE_BUFSIZE) && defined(MAX_VIRT_SECTOR_SIZE)
+#define DC_CACHE_BUFSIZE MAX_VIRT_SECTOR_SIZE
+#endif
+
+#if !defined(DC_CACHE_BUFSIZE) && defined(MAX_VARIABLE_LOG_SECTOR)
+#define DC_CACHE_BUFSIZE MAX_VARIABLE_LOG_SECTOR
+#endif
+
+#if !defined(DC_CACHE_BUFSIZE)
+#define DC_CACHE_BUFSIZE SECTOR_SIZE
 #endif
 
 #endif /* FS_DEFINES_H */