virtualbox.org

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).

Showing me a syntax example doesn't allow me to check your syntax. For that I need to see exactly what you typed. If this puzzles you then I'm referring to your use of "...." to substitute for the actual file path. That would not be a legal command.

Assuming you configured it correctly (media manager shows disk type: immutable), then next time you restart the VM, data from any previous session will be gone. However: suspend/resume is not a restart. Resetting the guest OS is not a VM restart.