diff --git a/user_io.cpp b/user_io.cpp
index 873dcc2..a77a287 100644
--- a/user_io.cpp
+++ b/user_io.cpp
@@ -40,6 +40,7 @@ static char core_path[1024] = {};
 static char rbf_path[1024] = {};
 
 static fileTYPE sd_image[4] = {};
+static int      sd_image_cangrow[4] = {};
 static uint64_t buffer_lba[4] = { ULLONG_MAX,ULLONG_MAX,ULLONG_MAX,ULLONG_MAX };
 
 static int use_save = 0;
@@ -1521,6 +1522,8 @@ int user_io_file_mount(const char *name, unsigned char index, char pre)
 	int ret = 0;
 	int len = strlen(name);
 
+	sd_image_cangrow[index] = (pre != 0);
+
 	if (len)
 	{
 		if (!strcasecmp(user_io_get_core_name(), "apple-ii"))
@@ -2680,14 +2683,20 @@ void user_io_poll()
 					{
 						// ... and write it to disk
 						__off64_t size = sd_image[disk].size >> 9;
-						if (size && size >= lba)
+						if (size && lba <= size)
 						{
 							diskled_on();
 							if (FileSeekLBA(&sd_image[disk], lba))
 							{
-								if (FileWriteSec(&sd_image[disk], buffer[disk]))
+								int sz = 512;
+								if (!sd_image_cangrow[disk])
 								{
-									if (size == lba)
+									__off64_t rem = sd_image[disk].size - sd_image[disk].offset;
+									sz = (rem >= 512) ? 512 : (int)rem;
+								}
+								if(sz && FileWriteAdv(&sd_image[disk], buffer[disk], sz))
+								{
+									if (size == lba && sd_image_cangrow[disk])
 									{
 										size++;
 										sd_image[disk].size = size << 9;