I believe this is related to the USB disk type
you can only set offline to 'fixed' USB disk type
if your USB disk is of type 'removable' the set offline option is not available
so
- USB removable uses unmount/mount
- USB fixed (more like internal HDD/SSD) uses offline/online - hyper-v needs disk/flash disk of this type
maybee you remember it from the days of windows-to-go
some vendors offered support tools for converting the disk type for specific USB drives but this was not common feature
solution in your case would be, to use USB disk type of 'fixed'
regards