I have Ubuntu as the guest installed and marked the appropriate vdi as immutable:
UUID: 6dafe8ee-081f-4428-8012-cc9b7caf8af2
Parent UUID: base
State: locked read
Type: immutable
Location: /Volumes/..../file.vdi
Storage format: VDI
Format variant: dynamic default
Capacity: 8192 MBytes
Size on disk: 6074 MBytes
Child UUIDs: e57c52cd-bc07-43bc-b5d7-1deb400f183e
via
VBoxManage modifyhd /Volumes/..../file.vdi --type immutable
If I create a file within the guest, the file is still exist after reboot or reset the virtualmachine. I thought, each reboot/reset discards every change (e.g. create file).