NAS326 defaults after restart or shutdown

Your problem is not exactly the same. znas4home had a bad block in the middle of the 'Config' partition. Your bad block is inside 'RootFS-1', and the bad PEB's are in 'RootFS-1' and 'RootFS-2'. Those partitions are only used when starting with fresh disks, or when updating firmware.

In your case there is a strange sequence when attaching the 'Config' partition:

UBI: attaching mtd2 to ubi2
UBI: scanning is finished
UBI: attached mtd2 (name "Config", size 10 MiB) to ubi2
UBI: PEB size: 131072 bytes (128 KiB), LEB size: 126976 bytes
UBI: min./max. I/O unit sizes: 2048/2048, sub-page size 2048
UBI: VID header offset: 2048 (aligned 2048), data offset: 4096
UBI: good PEBs: 80, bad PEBs: 0, corrupted PEBs: 0
UBI: user volume: 1, internal volumes: 1, max. volumes count: 128
UBI: max/mean erase counter: 2/1, WL threshold: 4096, image sequence number: 1497890039
UBI: available PEBs: 0, total reserved PEBs: 80, PEBs reserved for bad PEB handling: 40
UBI: background thread "ubi_bgt2d" started, PID 1566
UBIFS: background thread "ubifs_bgt2_0" started, PID 1570
UBIFS error (pid 1568): ubifs_recover_master_node: failed to recover master node
UBIFS: background thread "ubifs_bgt2_0" stops
UBI: detaching mtd2 from ubi2
UBI: mtd2 is detached from ubi2
UBI: attaching mtd2 to ubi2
UBI: scanning is finished
UBI: empty MTD device detected
UBI: attached mtd2 (name "Config", size 10 MiB) to ubi2
UBI: PEB size: 131072 bytes (128 KiB), LEB size: 126976 bytes
UBI: min./max. I/O unit sizes: 2048/2048, sub-page size 2048
UBI: VID header offset: 2048 (aligned 2048), data offset: 4096
UBI: good PEBs: 80, bad PEBs: 0, corrupted PEBs: 0
UBI: user volume: 0, internal volumes: 1, max. volumes count: 128
UBI: max/mean erase counter: 0/0, WL threshold: 4096, image sequence number: 1899997034
UBI: available PEBs: 36, total reserved PEBs: 44, PEBs reserved for bad PEB handling: 40
UBI: background thread "ubi_bgt2d" started, PID 1589
UBIFS: default file-system created
UBIFS: background thread "ubifs_bgt2_0" started, PID 1594
UBIFS: mounted UBI device 2, volume 0, name "ubi_config"
UBIFS: LEB size: 126976 bytes (124 KiB), min./max. I/O unit sizes: 2048 bytes/2048 bytes
UBIFS: FS size: 3301376 bytes (3 MiB, 26 LEBs), journal size 1015809 bytes (0 MiB, 6 LEBs)
UBIFS: reserved for root: 155931 bytes (152 KiB)
UBIFS: media format: w4/r0 (latest is w4/r0), UUID 0A079606-A8CB-43EB-9EDA-C4A58341FB0D, small LPT model

It attaches twice. The first attempt errors out on 'UBIFS error (pid 1568): ubifs_recover_master_node: failed to recover master node'. The second one reports: 'UBI: empty MTD device detected' and 'UBIFS: default file-system created'. So apparently the error causes the UBI system to reset the whole partition.

This is reproducible? Have you already tried a proper factory reset? A factory reset formats the Config partition in another way as the error handling does. I think.

[quote]I removed the disks <snip> and then on reboot it freezed[/quote]

That could be related to the bad block in rootfs 1. When I said it is only used on a fresh installation or firmware update, I was not complete. The partition contains a gzipped filesystem which contains a major part of the firmware, among which the webinterface. Normally it's decompressed to the harddisk once. On next boot it uses the filesystem blob on harddisk. (Nand is slow to read, so boottime is significantly shortened by this cache).

When booting without disks, the firmware creates a ramdisk in which the gzipped file is extracted. But of course that fails if the file is corrupted. In that case there is no webinterface, nor ssh. I think the firmware will set the flag to use the other rootfs next boot.

Maybe some aggressive erasing will do the trick?

su
umount /etc/zyxel
ubidetach -m 2
flash_eraseall /dev/mtd2

And else, as the vanilla kernel supports the NAS326, provided you pass the right dts file, in theory you can install any Linux distro, provided it supports headless and Armv7. Arch and Debian come to my mind. You mention 'on USB', but there is no reason why you couldn't install it on harddisk. The only limitation is that u-boot needs to be able to find&load uImage and uInitrd.

I'm not aware of any available NAS os, except OMV, which is basically a Debian package.