[Quoth]

I find that the file system has exclusive MTP on some versions of Linux desktop.
On some devices doing an eject solves it.
But I go
System Monitor -> Processes -> scroll to "gvfs-mtp-volume-monitor" Right Click: Stop Process
Then Calibre works with the Boyue Likebook Mars perfectly.

You need to Right Click and "Continue Process" to have File Manager access when you arn't using Calibre, maybe to copy some non-book files or something.


MTP is horrible. Only one program at a time can use it. This is an MTP issue, not Linux. Same problem in Windows, such as connecting an audio library / player. By default Explorer gets it.

Warning:
On Linux: Don't end or Kill "gvfs-mtp-volume-monitor" as it will be automagically restarted. Some desktops/filemanagers allow disable of MTP FS and then Calibre works. I usually use Mint Linux 20.1, Mate desktop (Marco) and Caja File manager. That combo doesn't let Calibre access MTP devices and the Control Panel settings to disable MTP Filemanager only work on other desktops. Maybe KDE. I have usually 2 or 3 desktop managers and 2 or 3 accounts on every Linux I have in case of accidents and to test issues like this.

It's not USB permissions.