ReadyNAS keeps freezing during Backup

Hi,

for some time I had no problems with my ReadyNas 104. But today I was reconfiguring my setup - concerning USB-device - and keep getting problems.

The main problem is: I keep getting the message "__percpu_counter_add+3c", but I am going to tell the whole story to understand the whole history of the problem:

I have connected a Fantec QB-35US3 with 4 disks connected to the first USB connector at the rear of the NeatGear. Until now with no real problems. The four drives I had up til now had the following capacity: 500 GB, 1 TB, 2 TB and 5 TB.
All four drives are shown after connecting the device. They are connected as four USB devices and I can use them via network with no problems.

Now I bought another 5 TB drive and the problems began. First I switched the 500 GB drive with the new 5 TB. Reconnecting the USB drive I only got one 5 TB drive shown. Rebooting gave the same results - except that I got sometimes the other 5TB drive, but not both at once!

After some searching in the forums I found a possible solution for that: So I upgraded the Firmware from 6.6.0 to 6.6.1.

Result: Now no USB drive was recognized. Even rebooting did nothing. So I connected the Fantec to the other USB on the rear of the ReadyNAS and... it worked... BUT again only one 5TB drive was recognized. Same problem as before.

Next idea: I have a second Fantec casing and switched to that. At first I only put in the two 5 TB drives. And that worked! So I put in two more drives with 3 TB and everything looked fine. All four drives are shown and seem ready to use. I don't get it why it's working - but okay. I'm happy. So I thought!

Now I wanted to start my backup and I keep getting the above mentioned error message in the ReadyNAS display: "__percpu_counter_add+3c"
The backup starts copying some files at first, but then it is stopping. The ReadyNAS is freezing. To reactivate it I have to pull the power cord. Reconnecting the device I get the message that there is a problem and I don't get write capability. So I connected the Fantec to my PC and I got the message that the file system is corrupt. After repairing it I could reconnect it to the ReadyNAS but I get the same error when I start the Backup.

I tried copying via ReadyNAS internal backup schedule and using a pc via network. Both result in the same error.

Using the USB drive directly at my PC everything works fine. I'm doing my backup now using the PC via network. But that's not the way I wanted it.
All drives are formatted in NTFS.

Any ideas what's the problem?