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Connection problems. ata5 mostly in logs, but also some ata4, maybe others.
 
These are on the motherboard controller.
 
Check all connections, all disks, both ends, SATA and power, including splitters.
 
Then try again and post new diagnostics.

Was the disk disabled before the rebuild? Or was it just unmountable? Or both? These are different and independent conditions.
 
Rebuild will fix a disabled disk, but it won't fix an unmountable filesystem. And it's much better to repair the filesystem before attempting to rebuild (if disabled), especially if rebuilding onto the same disk.
 
Check filesystem on disk3 from the webUI and post the results.
 
 

You will have to set a New Config.
 
https://docs.unraid.net/unraid-os/manual/storage-management/#reset-the-array-configuration
 
Note that SSDs in the array cannot be trimmed.
 

You want these all off the array so Dockers/VMs will perform better and so array disks can spin down since these files are always open.
 
They can't be moved while Docker/VM Manager is enabled. So you can't enable them again until the move is complete. By default, mover runs once per day in the middle of the night. You can manually invoke Mover in Main - Array Operations.

Does it have a unique GUID? 

The config folder from flash is all you need to get all of your configuration going again on a new flash drive.
 
If you can't get the config folder
 
https://docs.unraid.net/unraid-os/manual/changing-the-flash-device/#what-to-do-if-you-have-no-backup-and-do-not-know-your-disk-assignments

Copy config folder from the download zip, or just recreate the flash. 
 
All of your configuration is in the config folder, including your disk assignments and your license. Are you sure you wanted to get rid of all of that?
 
Dynamix Factory Reset plugin is a better approach. 

yes. Very much so.
 
Makes me wonder if you have others you should be concerned with.
 
Attach Diagnostics to your NEXT post in this thread.

https://docs.unraid.net/unraid-os/manual/changing-the-flash-device/

You can simply replace the smaller drive with the larger drive and rebuild, assuming you have parity.

You can go directly to the correct support thread for this container by clicking its icon in your Unraid webUI and selecting Support.

https://docs.unraid.net/unraid-os/manual/shares/user-shares/#moving-files-from-the-array-to-a-pool-cache

Either edit your existing template to include the new fields, or edit a new template to include your specific fields. Don't think there is any other way.

You can use the Include setting for each share to assign your drives to your share and his drives to his share. All would be protected by parity. Single parity will allow one failed disk, whatever share. 

You have too many missing disks. What happened to disks 3 and 4? They aren't seen in your diagnostics or you screenshot.

You can just copy the data with Unassigned Devices to the USB disks. No advantage to making them an array and likely some disadvantages with USB in the array. 

Gbit degrades to 100M if a signal pin is missing somewhere along the way. Check cables, ports, plugs on router, switches, computers.

New Config - Retain All.
 
Before starting array, make any assignment changes you want. Be sure to leave parity assignments as they are.
 
Check parity valid box and start the array.

https://docs.unraid.net/unraid-os/manual/shares/user-shares/#primary-and-secondary-storage-unraid-612

Connection problems multiple disks. Probably power. Any splitters?

https://docs.unraid.net/unraid-os/manual/changing-the-flash-device/#what-to-do-if-you-have-no-backup-and-do-not-know-your-disk-assignments