diff --git a/SDBlockDevice.cpp b/SDBlockDevice.cpp
index 3dc70cc..22576d1 100644
--- a/SDBlockDevice.cpp
+++ b/SDBlockDevice.cpp
@@ -941,7 +941,7 @@ uint32_t SDBlockDevice::_sd_sectors() {
                 _erase_size = BLOCK_SIZE_HC;
             } else {
                 // ERASE_BLK_EN = 1: Erase in multiple of SECTOR_SIZE supported
-                _erase_size = ext_bits(csd, 45, 39);
+                _erase_size = ext_bits(csd, 45, 39) + 1;
             }
             break;