Since upgrading my Mac to BigSur on start up I get the error message The Disk "Macintosh HD_Data" Can't be unlocked. I accept it and all seems to work OK. How can I stop this happening

Open Disk Utility and look at the list of volumes. You should see Macintosh HD and Macintosh HD - Data.

if there are more Data volumes, select each and look at the Mount Point in the info pane. The good one is mounted at /System/Volumes/Data.

Select each of the others and click the Remove Volume ( – ) button.

Bry369 wrote:

Since upgrading my Mac to BigSur on start up I get the error message The Disk "Macintosh HD_Data" Can't be unlocked. I accept it and all seems to work OK. How can I stop this happening

if you have Two Macintosh HD - Data

from Disk Utility.app you can see the mount point

One will be mounted at /System/Volumes/Data this is the one you want to keep,

The other will be mounted at /Volumes and you can simply use the “ -“ to delete it.

ref: About the read-only system volume in macOS Catalina - Apple ...

https://support.apple.com/en-us/HT210650

here is an example of a properly formatted drive:

The gray one is likely encrypted which is why it wasn’t mounted. I think you can still remove it.

I only see the good one. the other one is greyed out and says "not mounted"

Post a screenshot. I don't think you are looking at the Data volume.

If it is Not Mounted, you can just remove it, but I don't know if that is really what you are looking at. You should see Macintosh HD grayed out, but that is not a Data volume.