It seems that making recording on TV and then deleting them leaves files behind, e.g. .im.
After 700 files in the directory the TV starts to get very slow.
After the directory reaches some 800 files the TV software starts giving random errors. Recording still works but the recordings list is no longer available due to error. The recordings are still available through Source -> USB.
Here is a software update request:
1) remove all files related to a recording after recording has been deleted.
2) remove all excess files from the directory periodically
3) create and update an index file every time a recording has been added or removed and use it instead of scanning the directory when opening recordings
and to other users struggling with this problem, the first time I was able to plug the drive into windows, use filename as timestamps to check which timestamps have no data content and remove the files with no problems. The second time I did this I lost all the recordings. Maybe the NTFS defrag was too much for the tv.