Update secure deletion guidelines

guide.md

@@ -8169,6 +8169,7 @@ So, you want to be sure. To achieve 100% secure deletion on an SSD drive, we wil
 -   Easy options for less experienced users:
 
     -   If available, just use the Secure Erase option available from your BIOS/UEFI (ATA/NVME Secure Erase or Sanitize).
+        - It's worth noting that this relies on your drive's firmware. Some drive manufacturers have messed up the implementation, causing data to still be recoverable.
 
     -   Just re-install a fresh operating system (delete/quick format the drive) and re-encrypt it. The full disk encryption process should erase all previous data from the disk.
 
@@ -8176,6 +8177,20 @@ So, you want to be sure. To achieve 100% secure deletion on an SSD drive, we wil
 
 -   Technical options for more advanced users:
 
+    -   Overwrite the entire drive's contents
+        -   HDDs:
+            -   Overwrite the drive's contents using a tool like [srm](https://www.howtogeek.com/425232/how-to-securely-delete-files-on-linux/), [wipe](https://linux.die.net/man/1/wipe), [shred, etc.](https://recoverit.wondershare.com/harddrive-tips/format-and-wipe-linux-disk.html). Ideally you want to use the Gutmann method, which was created for most effective data erasure on all drives. This method also works on SSDs, although it is overkill.
+            -   Simply overwriting the drive's contents is not always enough. Dedicated secure deletion tools are designed to perform multiple passes to more effectively wipe data. This is expecially important on older drives. I recommend using either `wipe` or `srm`.
+                - If using `wipe`, just use its default options (`wipe /dev/sdX`), as the defaults are tuned to most effectively wipe data on HDDs.
+                - If using `srm`, make sure to manually specify that it should perform a Gutmann wipe (`srm -G /dev/sdX`).
+        -   SSDs:
+            -   Overwrite the drive's contents. Tools like wipe or shred are often overkill, as they perform up to 35 passes. While they work, most SSDs require no more than a couple passes.
+            -   Use `wipe` with only a couple passes: `wipe -qQ2 /dev/sdX` (`-qQ2` means to passes. Replace `2` with the desired number of passes)
+            -   Use `srm` with a 3-pass overwrite: `srm -P /dev/sdX`
+            -   Use `dd`: `dd if=/dev/urandom of=/dev/sdX bs=8M status=progress conv=fsync`. This command will overwrite the drive with random data. To perform multiple passes (I recommend at least 2), simply run the command again until you're satisfied.
+                - The reason you run it twice is because SSDs have hidden ("overprovisioned") storage which can contain remnants of deleted data. Wiping twice forces the drive to wipe its overprovisioned storage. This is only guaranteed to work if each pass writes different data (which is why we wipe with random data on each pass).
+                - `bs=8M` writes 8MiB blocks at a time. This doesn't affect the quality of the data deletion, but adjusting it could affect how long it takes to wipe the drive.
+
     -   ATA/NVMe Secure Erase: This method will remove the mapping table that keeps track of allocated data on the storage Blocks but does not destroy the actual data.
 
     -   ATA/NVMe Sanitize Crypto Scramble (aka Instant Secure Erase, Crypto Erase), which applies to self-encrypting SSD drives: This method will change the encryption key of the self-encrypting SSD drive and render all the data stored in it unreadable.