Add ability to use random data or zeroes

README.md

@@ -6,8 +6,9 @@ device after it has been cleanly removed from its pool.
 
 ## What It Erases
 
-The script performs 10 overwrite passes with zeroes over the first 4 MiB and
-the last 4 MiB of the target block device. These regions include:
+The script performs 10 overwrite passes over the first 4 MiB and the last
+4 MiB of the target block device. It uses zeroes by default and can use random
+data when requested. These regions include:
 
 - MBR data and GPT primary partition table at the beginning of a disk.
 - GPT backup partition table at the end of a disk.
@@ -41,6 +42,17 @@ chmod +x erase-zfs-metadata.sh
 sudo ./erase-zfs-metadata.sh /dev/sdX
 ```
 
+Use `--source random` to overwrite with data from `/dev/urandom` instead of
+the default zeroes:
+
+```bash
+sudo ./erase-zfs-metadata.sh --source random /dev/sdX
+```
+
+The only accepted source values are `zeroes` and `random`. Random overwrites
+are typically slower and do not erase additional parts of the device; both
+modes overwrite the same first and last 4 MiB regions.
+
 Replace `/dev/sdX` with the actual device, such as `/dev/sdb`,
 `/dev/nvme0n1`, or a device-mapper path. Do not use the disk containing the
 running system.