Dear Florian
Thanks for writing us.
Sorry for the inconvenience caused.
We have checked your kernel logs thoroughly, we found the drives been failed unfortunately.
As you know that Raid-5 can sustain with 1 drive failure but in your case 3 drives (Drive 2,3,4) were having issues in Raid-5. Out of 3 drives, 2 drives (Drive 3 & 4) failed completely.
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Initially Drive 4, 3 and 2 are having issues.
Logs snippet:
4408, 0,2016-06-06,22:48:26,System,127.0.0.1,localhost,Host: Drive4 Disabled NCQ since timeout error., 0,1465246104,
4409, 0,2016-06-06,22:48:28,System,127.0.0.1,localhost,Host: Drive3 Disabled NCQ since timeout error., 0,1465246108,
4410, 1,2016-06-06,22:48:30,System,127.0.0.1,localhost,Host: Drive2 Read I/O error, sense_key=0x0, asc=0x0, ascq=0x0, CDB=88 00 00 00 00 01 70 67 89 58 00 00 01 30 00 00 ., 0,1465246110,
Then Drive 3 failed because of this reason, Raid-5 became Degraded.
Logs snippet:
4414, 2,2016-06-06,22:48:55,System,127.0.0.1,localhost,Hot-remove Host: Drive3 failed., 0,1465246135,
4415, 1,2016-06-06,22:48:56,System,127.0.0.1,localhost,[Pool 1] RAID Group 1 is in degraded mode., 0,1465246136,
After then, Drive 4 with lots of Write I/O hence Raid-5 group cannot read and write on this drive as well hence system automatically detected this condition and mounted this Raid group as "degraded and read-only mode." . This is Special condition, wherein you cannot recover the Raid but system let the user to take only the backup.
Logs snippet:
4422, 1,2016-06-06,22:49:13,System,127.0.0.1,localhost,[Pool 1] RAID Group 1 is in degraded and read-only mode., 0,1465246153,
4423, 1,2016-06-06,22:49:15,System,127.0.0.1,localhost,[Volume RAID5, Pool 1] Mount the file system read-only., 0,1465246155,
Unfortunately drive 4 also completely failed so Raid-5 cannot handle 2 drive failure hence it became inactive mode.
Logs snippet:
4430, 2,2016-06-06,23:03:31,System,127.0.0.1,localhost,[Pool 1] RAID Group 1 is inactive., 0,1465247011,
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In this case, 3 drives are having issues. Raid-5 can sustain one drive failure but 2 drive failed and 1 drive error out. Sorry, it is not possible to recover and you have to replace those drives, delete the config and start from scratch.
HDD smart test cannot test the SMART values of the drives, if the drives are not completely accessible because of I/O error. You can do the scan drive on those disks, it can show the real identity of the drive status.
If not, you can test with HDD vendor tools to know the exact status of the drives.
WD Data Lifeguard Diagnostics : http://support.wdc.com/KnowledgeBase/answer.aspx?ID=940
Seagate Sea tools: http://knowledge.seagate.com/articles/en_US/FAQ/202435en
Note: one more suggestion, please use the compatible UPS to your NAS model. Your exisiting UPS "American Power Conversion Back-UPS BR 900GI" device is not compatible to your NAS model.
Please find the compatibility list here: https://www.qnap.com/i/in/compatibility/?model=21&category=9
If you have queries, please feel free to write us. Thanks!