per PeterV suggests, I move some recording to a sub-folder, LG TV happily ignors anything in sub-folder, and behaves well with less recorded clips.
But I have to correct myself that the maximum number it can handle is only around 170 only (instead of what I posted previously of ~400) beyond that, the time shift function sometimes work, and jump back to real time from time to time (i.e. all shifts are gone) The recorder sometimes save part of the show and terminates with an error., but occasionally it success. TV never says anything like disk full or definitely an error with something specific (like too many clips etc) , just acted weird. (my 1TB disk has only around 400MB data and only files saved by TV)
So my conclusion is too big a hard disk is pointless with buggy firmware.
As a side note: if you move back recorded clips back to lg_tv folder, it will be display again correctly. So there is not an index file you have to worry about. So in case you really want to keep something offline, just copy them to somewhere else. But recorded clips only uses number as names, you cannot easily identify which is which. The only thing you can rely on is the time stamp of the folder. No names nor channel info.