ultra 6 plus limited to 10 Tio on line

no.

if your expansion fails, its because of bad disk(s)

quite a few people are using ultras/pro with 6x 3tb

got the answer, online expansion is possible by 8TB maximum for lifetime of the ext4 filesystem, maybe you hit this border.
official answer is to backup/factory reset/restore, but I was able to do a offline expansion.
viewtopic.php?f=51&t=51475&p=294588#p294588

Thanks for your answers! I try a disk swap between hdd 5 and hdd 6 ; if bad disk, I will get a warning on it!

since dead status on channel 5 after swapping, I did a scan with hddregenerator, 99.9% scan without errors, then stuck, extracting disk resulted with a BSOD...

First - which WD Caviar Green 3TB did you install in your Ultra 6? Of the three models listed on Newegg (all 3TB Green) only one is on the HCL.

Second - you are supposed to run the vendors test, not a third party test. The vendors test has specific tests for their drives. Did you download Data Lifeguard from WD and try it?

Third - A BSOD on your PC cannot be caused by the NAS. I am assuming that it was either the drive or the testing software.

the drive is wd30EZRX ; I did not test life guard, cause, if unable to access sectors with hddregenerator, lifeguard would do no miracles ; I am not pretending that the nas caused a BSOD, hdd was in an ICY DOCK!!!

The WD30EZRX is on the HCL. As to the ICY Dock, I'm not sure which one you used or how it was connected to your PC, but generally to properly test the drives, you do need to use the testing software as written by the drive manufacturer and while the drive is attached directly to a MB SATA port, not an external adapter or enclosure connected via USB or even eSATA.

Data Lifeguard will read the parameters and test the drive using routines and tests developed by the manufacturer. While I am far more familiar with SeaTools than I am with Data Lifeguard, I do know that SeaTools will not test a drive connected via any enclosure as the electronics in the enclosure interfere with the tests. The long test on SeaTools also takes about two hours to run on a 1TB drive (I have never run it on a 3TB).

BTW - I have never had a BSOD from either Data Lifeguard or SeaTools.

While the vendors tools can in some instances "repair" a drive, most often it is simply to test the drive to determine if it has failed in some manner. In the case of SeaTools, if the Seagate drive fails the long test, you get a report that gives you a failure code that must be used when sending a drive back for replacement. Until a drive is tested with the vendor tools it is difficult to determine if it is the drive, the configuration or the NAS hardware. Testing is used to resolve the items that have not caused the problem to determine what has caused it.

Western send me another hdd ; reset factory plus RAIDiator 4.2.18 gives 13.6 Tio now...