You can use NIUBI Partition Editor Free Edition to convert Dynamic Disk to Basic without losing data on Windows 11/10. All features discussed.

Not only was I able to backup the data, but I was also able to convert the drive from Dynamic to Basic using the tool. Great find. I recovered a good 120GB worth of work that I have now backed up.

If you want to convert a VirtualBox Fixed Disk to a Dynamic Disk or vice versa, follow the instructions mentioned in this guide.