Compact VirtualBox guest disks.

virtualbox.md

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+# VirtualBox
+
+## Compact Windows disk
+1. on Windows guest system:
+```
+.\sdelete.exe c: -z
+```
+2. on Linux host system:
+```
+vboxmanage modifymedium --compact /path/to/vm/disk.vdi
+```
+
+Note: disk size will grow during the `sdelete.exe` process; then, it can be compacted.
+
+Note: if you don't have enough disk space to allow the expansion, just do it in multiple steps, by stopping `sdelete.exe` as soon as the host's disk is almost full.