Have an external drive set to be scanned in Everything Options > Indexes > NTFS. Let's call this drive O:. Disconnect this drive for now.
Have a folder set via Everything Options > Indexes > Folders. It would help if the folder is a mapped network share that takes a while to scan. Let's call this drive S:.
Go to the Folders option and click Rescan Now. Everything starts Rescanning. Leave the Options window open so that you can see that...
Now plug in drive O:.
What will happen is that when drive O: is connected Everything will start scanning it (which will be very quick), but in doing so, it will abandon the requested rescanning operation of drive S:. This is no indication to the user that the S: drive is not fully rescanned.
The expected result is that either Everything continues to rescan the drive S: (folder) when it scans drive O:, or it will pause S: and resume it when it is done with O:, depending on how multi-threaded the scanning of different drives/folders is.