diff --git a/firmware/target/arm/rk27xx/sd-rk27xx.c b/firmware/target/arm/rk27xx/sd-rk27xx.c
index a49e8e5c43..02180d9faa 100644
--- a/firmware/target/arm/rk27xx/sd-rk27xx.c
+++ b/firmware/target/arm/rk27xx/sd-rk27xx.c
@@ -464,12 +464,15 @@ int sd_read_sectors(IF_MD(int drive,) sector_t start, int count,
     int cnt, ret = 0;
     unsigned char *dst;
 
-    mutex_lock(&sd_mtx);
-    enable_controller(true);
-
+    /* Checked before taking the lock: returning from after it left sd_mtx
+     * held and the controller powered. With no card numblocks is 0, so
+     * that was every request. */
     if (count <= 0 || start + count > card_info.numblocks)
         return -1;
 
+    mutex_lock(&sd_mtx);
+    enable_controller(true);
+
     if(!(card_info.ocr & (1<<30)))
         start <<= 9; /* not SDHC */
 
@@ -602,12 +605,15 @@ int sd_write_sectors(IF_MD(int drive,) sector_t start, int count,
     unsigned char *src;
     /* bool card_selected = false; */
 
-    mutex_lock(&sd_mtx);
-    enable_controller(true);
-
+    /* Checked before taking the lock: returning from after it left sd_mtx
+     * held and the controller powered. With no card numblocks is 0, so
+     * that was every request. */
     if (count <= 0 || start + count > card_info.numblocks)
         return -1;
 
+    mutex_lock(&sd_mtx);
+    enable_controller(true);
+
     if(!(card_info.ocr & (1<<30)))
         start <<= 9; /* not SDHC */