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As you can see if you mouseover the download link, it should be .exe file. Make sure your browser isn't blocking the download.

You shouldn't even try to run any computer unless memory is working perfectly. Everything goes through RAM, the OS and other executable code, your data, everything. The CPU can't do anything with anything until it is loaded into RAM.

The only acceptable result for memtest is no errors.

So 4 drives sharing a single USB connection, and another 4 drives sharing another single USB connection. I think it likely that parity will only complicate things for you. And parity is not a substitute for backups anyway. Do you have good backups?

Tell us more about these enclosures. Can you post a link to the product?

How are these "enclosures" accessed by the computer? USB or SATA or SAS?

You should be able to examine most of your config in a text editor. super.dat (array assignments) is not simple text, and many plugins will have .txz files in their folders, but the .cfg files at the top of config on the flash drive should be simple text. See if any look corrupt.

Looks like you are having connection problems with that drive. In fact, I don't even see a SMART report for that device, so it must have completely disconnected. Check connections, both ends, SATA and power, including splitters.

Parity does not contain any of your data. A single parity drive provides redundancy for all array drives. Parity is basically the same concept wherever it is used in computers and communications. Parity is just an extra bit that allows a missing bit to be calculated from all the other bits. It is a very simple calculation. See here: https://docs.unraid.net/unraid-os/getting-started/what-is-unraid/ Then expand the Software-defined NAS section, then expand the How Parity Works section

You should have added it to the pool then reformatted. Now that it is in the pool try formatting it.

You can have one or two parity. They are independent, and each must be a single drive at least as large as the largest single data drive. You mention an enclosure and adding another. USB not recommended for assigned drives for many reasons.

Do you have an attached keyboard and monitor? Can you access the Unraid command line?

https://docs.unraid.net/unraid-os/system-administration/maintain-and-update/changing-the-flash-device/#backing-up-your-flash-device

https://docs.unraid.net/unraid-os/system-administration/maintain-and-update/changing-the-flash-device/#recovering-from-a-failed-boot-device-without-a-backup

Is that folder a ZFS dataset?

The filesystem on Unraid v5 was ReiserFS. It is no longer supported but it will still work in V6. In order to convert to XFS (recommended for array), and btrfs or ZFS (recommended for cache), you have to copy the data somewhere and reformat each disk.

Do you have a flash backup?

You have Minimum Free for that share set to 3.4 TB, so any writes to that share overflow to Secondary since Primary doesn't have that much free. https://docs.unraid.net/unraid-os/using-unraid-to/manage-storage/shares/#minimum-free-space After you fix the Minimum Free setting for that share, you should also consider Minimum Free setting for each of those pools. https://docs.unraid.net/unraid-os/using-unraid-to/manage-storage/cache-pools/#minimum-free-space-for-a-cache-pool

I should have said lots of reads from all other array disks. The only other array disks you have is parity and disk2, both are being read to calculate the emulated data to write to rebuilding disk1.

Trying to match old hardware might actually be more expensive due to availability.

This is the much more serious error FCP is telling you about (which you didn't mention): Sep 11 19:01:41 SoftServeUSA root: Fix Common Problems: Error: Invalid folder cache contained within /mntYou must have some container host mapping specifying a path to the (no longer existing) pool named cache. Or maybe it's something about the way you have Custom SMB Shares setup, I don't use that. Since cache isn't mounted storage, anything stored there is in rootfs, the RAM space reserved for the OS to run in. Anything stored there will not survive reboot, since it is in RAM. And anything stored there could use up all the space the OS needs and other problems could result.

Post a screenshot of the settings for the data user share.

Just some advice about using the forum. The reason I said is because when no new posts are made to the thread, it may not get visited again since it was already read. When you edit a previous post without making a new post, anyone that has already read the thread (and may be interested in helping you), may not go back to the thread since it has no new posts and so doesn't show up as an unread thread.

During rebuild, you should see lots of Writes to the rebuilding disk, lots of Reads of all other disks, zero Errors for all disks.

You can read (and even write) the "emulated" disk. Unraid isn't using the actual disk until rebuilt. It is emulating the disk from the parity calculation by reading all other disks to get the data of the disabled disk. Writes of the emulated disk update parity so any writes to the emulated disk can also be recovered by rebuilding. Parity works pretty much the same whatever version of Unraid and it is basically the same concept wherever it is used in computing and communications. Parity is just an extra bit that allows a missing bit to be calculated from all the other bits. It is a very simple calculation. If interested, go here: https://docs.unraid.net/unraid-os/getting-started/what-is-unraid/ then expand the Software Defined NAS section, then expand the How Parity Works section.