hard disk spaces wasted by *_S6Directed files

Probably none of them, without doing checks of the tasks you have on board, and then stopping BOINC and doing some careful editing of your state file (client_state.xml). If you delete files without adjusting your state file, BOINC will immediately download fresh copies.

The wc utility says that you have 526 of these files, no doubt covering quite a diverse range of frequencies. Thank you for not showing all of them :-).

The first thing to do is work out what particular frequencies are in use for current tasks. I took a look at both your hosts and they are working on frequencies around 741 - 752Hz and 835Hz respectively. So it should be fairly safe to consider removing data files for much lower frequencies than those - say around 500Hz or less. Certainly the small number you listed are in that category.

Grab a copy of /var/lib/boinc/client_state.xml and paste it somewhere you can play with it. Open the file with a text editor and browse through it looking for .... blocks containing data file names that are in the 350 - 500Hz range. These certainly should be 'not currently in use'. There will be one block for each of these old data files and each block will consist of a number of lines of information about the particular file. The formatting is very important so you should spend enough time examining things so that you understand exactly how things are laid out.

Since you are working with a copy, you can delete these blocks and it wont matter if you make a mistake or two while you are getting the hang of things. You should do this as a trial to make sure you can delete without disturbing the overall format of the file. Once you are comfortable with what you are doing, you can throw away this particular copy.

For the real thing, you need to stop BOINC first. You should note the ownership and permissions of the real state file so you can make sure nothing gets disturbed by the editing. Also, in case of problems while editing, you should save a backup copy of the file. You should carefully remove the blocks for the whole range of files you wish to delete and finally save the file when you are sure all is correct. Because you have quite a large number of files, this is potentially quite a tedious operation. Be careful, as there is no guarantee that the blocks will be in any particular order, although it's likely that quite a few will be in ascending order of frequency. If you make a mistake that can't be 'undone' you can always start again with a further copy of your backup copy. When you finish, you can restart BOINC and if you haven't made any editing mistakes, there should be no complaints.

At that point you can safely delete all of the data files for which the blocks have been removed. At some point in the future you could possibly get some of them back again if the scheduler decides to send you any further tasks that depend on particular frequencies you have deleted. If you have left enough 'margin' between current task frequencies and the ones you have deleted, that's probably not very likely.

One final point. The above is not standard procedure and if you happen to make a mistake in the editing, you could end up with quite a mess. I will not take responsibility if that happens. I'm giving the instructions in good faith but with no guarantee that I've completely covered every possible detail. If you don't fully understand the instructions, don't try to do it. Unless you are quite desperate to reclaim some disk space, you are much better off leaving things alone. Eventually, the old data files are supposed to be removed automatically, once the project determines that there are no further tasks that rely on data for those particular frequencies.