Transmission

Hi, I am using T 2.77 (14031) web ui on Asus RT-N65U router (custom firmware 3.0.3.3-046). My os is Win7 x64. I have my downloads on the external hard drive connected to the router. I had several downloads successfully complete and seeding for a couple of days. When I opened transmission today some of the torrents were showing "Error: No data found! Ensure your drives are connected or use "Set Location". To re-download, remove the torrent and re-add it." When I turned to the location where the files were I found only empty folders (named properly). So it looks just like those files disappeared from those folders. The only thing I found in the log is that T has changed its date and time to my system's earlier in the morning without any of my interference (which is kinda weird), here is that part:

Jan 1 00:00:21 kernel: Built_for__Asus_n65u_2012-03-29
Jan 1 00:00:21 kernel: ufsd: use builtin utf8 instead of kernel utf8
Jan 1 00:00:21 kernel: ufsd: sda1 without journal
Jan 1 00:00:27 FTP server: daemon is started
Jan 1 00:00:28 Samba Server: daemon is started
Jan 1 00:00:29 NTP Scheduler: Synchronizing time to pool.ntp.org ...
Apr 12 07:18:44 Transmission: daemon is started
Apr 12 07:18:50 NTP Scheduler: System time changed.
Apr 12 07:19:14 nmbd[553]: [2013/04/12 07:19:14, 0] nmbd/nmbd_become_lmb.c:become_local_master_stage2(392)
Apr 12 07:19:14 nmbd[553]: Samba name server SEAGATE is now a local master browser for workgroup WORKGROUP on subnet 192.168.1.1
Apr 13 07:18:40 NTP Scheduler: Synchronizing time to time.nist.gov ...

So my question is wtf happened? Where did some of my files go? Why did only a part of my files disappear? How do I prevent it in future?
Thank you all so much.

@CheerfulChe
Exact same thing happened to me!?
The folder where the files were stored is empty and the files are nowhere to be found?
I'm using T 2.5.1 on Ubuntu 12.04.
Not sure where to look for any logs?
The files are simply gone, not in Trash or anywhere else?
My mount point looks fine, unmounted and remounted, no difference.
Sorry for my bad manners, but seriously, wtf?
Thanks to anyone that can look into this!