I've got the same problems, I sometimes get the "unable to write to harddisk" error.
there is NO firewall or AV running, i have about 50gb of free space on my harddrive and everything else (e.g. µTorrent) works without any problems.
the error occures about 1-5 times a day, the download gets deactivated and the next download starts.
have had the prob with RS und Megaupload, seems not to be hoster-specific.
logfile:
it definitly IS a problem of jDownloader, not of any FW, AV or something like this. Please have a look at this
greez Undeath