Hi, thank you for the question.

The contents of the speed drop-down depends on what the device reports is possible to use on the media which is inside the drive. The selection is disabled because when the user is on that page, that information is not available yet (not until the burning process starts and the discs are loaded), so any random speed selected may turn out to be invalid.

For example, in the same burn you can include both CD and DVD discs. A 48x would be a valid speed for a CD-R blank, but not for a DVD rewritable one.

I hope this information is helpful.