It's this background thread: GetMissingAndUpdateFiles, which is a sub-task of the Import thread:
0007807: 14300: Import: Process: Starting import (Media Center 27.0.34 (64-bit))
0007808: 14300: Import: GetNewFiles: Start
0007898: 14300: Import: GetNewFiles: Finish (90 ms)
0007898: 14300: Import: GetMissingAndUpdateFiles: Start
0007898: 14300: Import: GetMissingAndUpdateFiles: Getting files
0007898: 14300: Import: GetMissingAndUpdateFiles: Sorting files
0007898: 14300: Database: CMediaFileIOLoad::Load: Loading: C:\Users\Rolf\AppData\Roaming\J River\Media Center 27\Library\Cache\File List Sort\Index.dat
0007900: 14300: Database: CDataHolder::Load: Field: Filename; Files: 4846; Pointer bytes: 39056; Data bytes: 32904; Elapsed ms: 1.308
0007902: 14300: Database: CDataHolder::Load: Field: Name; Files: 4846; Pointer bytes: 39056; Data bytes: 82056; Elapsed ms: 1.748
0007903: 14300: Database: CDataHolder::Load: Field: Artist; Files: 4846; Pointer bytes: 39056; Data bytes: 16488; Elapsed ms: 0.891
0007904: 14300: Database: CDataHolder::Load: Field: Album; Files: 4846; Pointer bytes: 39056; Data bytes: 16488; Elapsed ms: 0.958
0007905: 14300: Database: CDataHolder::Load: Field: Track #; Files: 4846; Pointer bytes: 9694; Data bytes: 16488; Elapsed ms: 0.887
0007907: 14300: Database: CDataHolder::Load: Field: Genre; Files: 4846; Pointer bytes: 9982; Data bytes: 16488; Elapsed ms: 0.871
0007924: 14300: Database: CMediaInfoArraySort::Sort: Files: 4829; Elapsed ms: 26.120
0007925: 14300: Import: GetMissingAndUpdateFiles: Looping 4829 files
0007936: 14300: Database: CDataHolder::Load: Field: Library Server URL; Files: 4846; Pointer bytes: 0; Data bytes: 72; Elapsed ms: 0.364
0007968: 14300: Database: CDataHolder::Load: Field: Date Modified; Files: 4846; Pointer bytes: 39056; Data bytes: 16488; Elapsed ms: 1.026
0007969: 14300: Database: CDataHolder::Load: Field: File Size; Files: 4846; Pointer bytes: 39056; Data bytes: 32904; Elapsed ms: 1.048
It's apparently checking if the files still exist. It doesn't import any new files, but checks existing ones.
Looks like that library Cache file still contains references to the files, even though you deleted them from the library.
Try this:
- backup your library, close MC
- delete that Cache file: C:\Users\Rolf\AppData\Roaming\J River\Media Center 27\Library\Cache\File List Sort\Index.dat
- test to check if the issue is gone
There's 1 or 2 other files in that folder, you can likely delete them as well.
If the problem persists and that Cache file is rebuilt, then it means those file references still exist somewhere in one of the Library files.
Only someone from MC team can tell us what exactly this GetMissingAndUpdateFiles thread is doing.