Once I safely eject from the computer, the Sage takes a few seconds but goes to the import screen/home screen depending on what I've been doing. I leave the USB cable connected due to paranoia about the Sage's battery depleting since I seldom check the battery level before connecting. To safely eject, I use calibre's SmartEject plugin since I use some of it's other features. The eject in the Device dropdown works as well if you are just looking to safely eject. Windows File Explorer eject also works but using the eject from the USB icon in the system tray does not.
The calibre
Check Books plugin lets you run epubcheck on multiple books but I just use the epubcheck plugin in either Sigil epub editor or calibre's ebook editor to check one at a time since that allows me to correct errors. I tried Check Books but the number of errors and having to refer back to fix the errors was a pain. Sigil with the epubcheck and ACE plugins just made it easier for me (you don't need ACE if you are not checking for accessibility).
You might just want to install NickelMenu and re-trigger the import from there. Most of the errors that epubcheck finds don't seem to have that much effect on the reading experience.