Hi there,
Generally, Storage Spaces on Windows cannot use USB flash drives.
The problem is the "Removable Disk" status of the flash drives. Windows don't let "Removable Disks" become part of the storage spaces pool. Thus this is all you need to solve, convert all your USB Flash drives from "Removable" to "Fixed".
You can try the below steps and see fi that helps you
-In Device Manager find the external drive under Disk Drives.
-right-click the drive and go to Properties
-Go to Policies
-select Better Performance
-select Enable write-caching on the device
-select Turn off Windows write-cache buffer flushing on the device
Once these steps are done check if you can create a storage pool using the USB.If you need to free up some space here are some suggestions https://support.microsoft.com/en-us/windows/free-up-drive-space-in-windows-a18fae02-a0fa-8df9-9838-8970f9939de4
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