Warning : I Restore one of my costumer 4 HDD Qnap Ts-412 (with RAID 5 conf) by this way, but This is my solution, it works perfect but still my advice is always request help from Qnap Taiwan.
One of our costumer told me that they change Qnap HDDs with new ones, but they forget that they have important files in old Qnap HDDs and asked me if there is anything we can do about this. Here is the guide how I solve this problem;
1 – First, power off Qnap, and Plug out all HDD’s.
2 – Start Qnap NAS without HDDs
3 – When Qnapfinder could find your NAS with “169.254.100.100″ Ip adress, Plug all HDDs randomly to your NAS device.
4 – Download Putty and Login to your Qnap (defauld Ip : 169.254.100.100 / User : admin / pass : admin)
At this point, if you check information about HDD one by one, you’ll see all HDDs seems in right orders, examle 2.th HDD is at 2.th slot, or 4.th HDD is at 4.th slot just like this table;
(HDD check command for 1.th HDD: mdadm -E /dev/sda3
HDD check command for 2.th HDD: mdadm -E /dev/sdb3
HDD check command for 3.th HDD: mdadm -E /dev/sdc3
HDD check command for 4.th HDD: mdadm -E /dev/sdd3)
5 – Now, type this command;
6 - Check one of these HDD with this command again;
#mdadm -E /dev/sda3
Now true HDD RAID order should be appear. At this case 1.st HDD (sda3) at 3.th HDD slot; and 3.th HDD (sdc) placed in 1.st HDD slot, so just change these HDD’s order physcally. Other two HDDs (sdb & sdd) are in right order, so no need to change their slots.
7 – Shut down Qnap, and restart without HDD’s. Login with Putty, Plug in HDD’s now with true HDD’s orders ant type these commands;
(before rebooting device, I can access whole datas by using winscp, so you can try winscp to recover some files)
8 – This should restore your system and Qnap should start without problem. But again must warn you about this is my solution and you may loose data or damage your RAID system.