I was wondering what all those "ocTransferId*.part" files on the server are used for and therefore if it´s a good idea / not critical to remove them.
From what I understand they are part of the full file (which usually exists next to the same file with "ocTransferId*.part" suffix), created while transfering from desktop clients to the server.
Such files should normally be deleted automatically. If they are more than 1-2 days old, I would check my cronjob if they are running properly or if they fail with an error (check logfiles). If you do this manually, you risk to delete files which are used for a current upload process.
For older files, it should be save to delete them. I generally put ownCloud in maintenance mode, and then move these files to a temporary location (in case they were important) and delete them from there after a certain time.
Cron runs fine. No error(s) in owncloud.log.
"sudo find /var/www/owncloud/data -name "*ocTransferId*.part" | wc -l" gives: 2940 files...
I checked the timestamp of those files using:
"sudo find /var/www/owncloud/data -name "*ocTransferId*.part" -printf '%Tc %p\n'"
Result: ALL of them (according their timestamp) seem to be from the day - or at least BEFORE that day, but none of them AFTER that day - when I disabled/removed OwnCloud encryption. Something went wrong that time (encryption key migration, don´t exactly remember).
So I deleted all of them using "sudo find /var/www/owncloud/data -name "*ocTransferId*.part" -delete"