diff --git a/libcheckisomd5.c b/libcheckisomd5.c
index bd7b86f..9f86734 100644
--- a/libcheckisomd5.c
+++ b/libcheckisomd5.c
@@ -67,7 +67,8 @@ static enum isomd5sum_status checkmd5sum(int isofd, checkCallback cb, void *cbda
     while (offset < total_size) {
         const size_t nbyte = MIN((size_t)(total_size - offset), buffer_size);
 
-        ssize_t nread = read(isofd, buffer, nbyte);
+        /** Read more than nbyte to make sure that it's O_DIRECT compatible. */
+        ssize_t nread = read(isofd, buffer, buffer_size);
         if (nread <= 0L)
             break;
 
@@ -76,9 +77,11 @@ static enum isomd5sum_status checkmd5sum(int isofd, checkCallback cb, void *cbda
          * size from where it started up to the end of the block it pre-fetched
          * from a cd drive.
          */
-        if (nread > nbyte) {
-            nread = nbyte;
+        if (nread > buffer_size) {
+            nread = buffer_size;
             lseek(isofd, offset + nread, SEEK_SET);
+        } else if (nread > nbyte) {
+            nread = nbyte;
         }
         /* Make sure appdata which contains the md5sum is cleared. */
         clear_appdata(buffer, nread, info->offset + APPDATA_OFFSET, offset);
diff --git a/libimplantisomd5.c b/libimplantisomd5.c
index 0688b1c..0f340a0 100644
--- a/libimplantisomd5.c
+++ b/libimplantisomd5.c
@@ -41,7 +41,7 @@ static int writeAppData(unsigned char *const appdata, const char *const valstr,
         return -1;
     }
 
-    memcpy(appdata + *loc, valstr, vallen);
+    memcpy(appdata + (APPDATA_OFFSET - BLOCK_SIZE) + *loc, valstr, vallen);
 
     *loc += vallen;
     return 0;
@@ -67,25 +67,29 @@ int implantISOFD(int isofd, int supported, int forceit, int quiet, char **errstr
         return -1;
     }
 
-    lseek(isofd, pvd_offset + APPDATA_OFFSET, SEEK_SET);
-    unsigned char appdata[APPDATA_SIZE];
-    if (read(isofd, appdata, APPDATA_SIZE) <= 0) {
+    lseek(isofd, pvd_offset + BLOCK_SIZE, SEEK_SET);
+    unsigned char appdata[BLOCK_SIZE * 2] __attribute__((aligned(BLOCK_SIZE)));
+    const size_t appdata_offset = APPDATA_OFFSET - BLOCK_SIZE;
+
+    /* Read more than needed to stay O_DIRECT compatible. */
+    if (read(isofd, appdata, BLOCK_SIZE * 2) <= 0) {
         *errstr = "Failed to read application data from file.";
         return -errno;
     }
 
     if (!forceit) {
         for (size_t i = 0; i < APPDATA_SIZE; i++) {
-            if (appdata[i] != ' ') {
+            if (appdata[appdata_offset + i] != ' ') {
                 *errstr = "Application data has been used - not implanting md5sum!";
                 return -1;
             }
         }
     } else {
         /* Write out blanks to erase old app data. */
-        lseek(isofd, pvd_offset + APPDATA_OFFSET, SEEK_SET);
-        memset(appdata, ' ', APPDATA_SIZE);
-        ssize_t error = write(isofd, appdata, APPDATA_SIZE);
+        lseek(isofd, pvd_offset + BLOCK_SIZE, SEEK_SET);
+        memset(appdata + appdata_offset, ' ', APPDATA_SIZE);
+        /* Write more than needed to stay O_DIRECT compatible. */
+        ssize_t error = write(isofd, appdata + BLOCK_SIZE, BLOCK_SIZE * 2);
         if (error < 0) {
             *errstr = "Write failed.";
             return error;
@@ -99,7 +103,6 @@ int implantISOFD(int isofd, int supported, int forceit, int quiet, char **errstr
     MD5_Init(&hashctx);
     char fragmentsums[FRAGMENT_SUM_SIZE + 1];
     *fragmentsums = '\0';
-
     const size_t pagesize = (size_t) getpagesize();
     const size_t buffer_size = NUM_SYSTEM_SECTORS * SECTOR_SIZE;
     unsigned char *buffer;
@@ -136,7 +139,9 @@ int implantISOFD(int isofd, int supported, int forceit, int quiet, char **errstr
         printf("fragmd5 = %s\n", fragmentsums);
         printf("frags = %lu\n", FRAGMENT_COUNT);
     }
-    memset(appdata, ' ', APPDATA_SIZE);
+    if (!forceit) {
+        memset(appdata + appdata_offset, ' ', APPDATA_SIZE);
+    }
 
     size_t loc = 0;
     if (writeAppData(appdata, "ISO MD5SUM = ", &loc, errstr))
@@ -183,12 +188,13 @@ int implantISOFD(int isofd, int supported, int forceit, int quiet, char **errstr
             appdata, "THIS IS NOT THE SAME AS RUNNING MD5SUM ON THIS ISO!!", &loc, errstr))
         return -1;
 
-    if (lseek(isofd, pvd_offset + APPDATA_OFFSET, SEEK_SET) < 0) {
+    if (lseek(isofd, pvd_offset + BLOCK_SIZE, SEEK_SET) < 0) {
         *errstr = "Seek failed.";
         return -1;
     }
 
-    ssize_t error = write(isofd, appdata, APPDATA_SIZE);
+    /* Write more than needed to stay O_DIRECT compatible. */
+    ssize_t error = write(isofd, appdata, BLOCK_SIZE * 2);
     if (error < 0) {
         *errstr = "Write failed.";
         return -1;
diff --git a/utilities.c b/utilities.c
index b00337f..c4a5bcc 100644
--- a/utilities.c
+++ b/utilities.c
@@ -41,7 +41,7 @@ static unsigned char *read_primary_volume_descriptor(const int fd, off_t *const
         return NULL;
     }
     unsigned char *sector_buffer;
-    sector_buffer = aligned_alloc((size_t) getpagesize(), SECTOR_SIZE * sizeof(*sector_buffer));
+    sector_buffer = aligned_alloc(BLOCK_SIZE, SECTOR_SIZE * sizeof(*sector_buffer));
     /* Read n volume descriptors. */
     for (;;) {
         if (read(fd, sector_buffer, SECTOR_SIZE) == -1) {
@@ -137,8 +137,6 @@ struct volume_info *const parsepvd(const int isofd) {
     result->offset = offset;
     result->isosize = isosize(aligned_buffer);
 
-    free(aligned_buffer);
-
     for (size_t index = 0; index < APPDATA_SIZE;) {
         size_t len;
         if ((len = starts_with(buffer + index, "ISO MD5SUM = "))) {
diff --git a/utilities.h b/utilities.h
index 909696a..50b2392 100644
--- a/utilities.h
+++ b/utilities.h
@@ -39,6 +39,7 @@ int getpagesize() { return 2048; }
 #define MAX(x, y) (((x) > (y)) ? (x) : (y))
 #define MIN(x, y) (((x) < (y)) ? (x) : (y))
 
+#define BLOCK_SIZE 512
 #define HASH_SIZE 32
 /* Length in characters of string used for fragment md5sum checking */
 #define FRAGMENT_SUM_SIZE 60UL