C6250 USB NAS STILL not working - 5 case numbers later and my support runs out tonight

Can't help much, however:

As long as the USB drive is not recognized by the router and does show up in the USB storage advanced settings, "Available Network Folders." there is not much one can do. The typical solutions (probably not all applicable in you case because it used work):

• Remove any other partitions from the USB storge device (e.g. boot or installation partitions e.g. with ISO images for whatever backup or security applications provided by the disk/enclosure maker - these can prohibit the recognition of the real data partition and file system especially on older ReadyShare router implementations) ... needless to say this will remove the content ... and re-create a single partition only, with a supported file system.
• Run a file system check, e.g. on a Windows system if NTFS is in place NTFS

After replugging the disk to the router, if this does bring back the USB device to be listed in the ReadyShare advanced Web UI you can continue.

When looking over the documentation (AC1600 WiFi Cable Modem Router Model C6250 User Manual), the implementation looks very basic (read: legacy). There are no controls for the SMB transport protocols (to enable/disable SMB 1.0/2.0/3.0 and the 3.0 features) - this brings me to the conclusion there is only an SMB 1.0 implementation in place:

• Search on Cortana (or the Windows search bar) for "Windows features". Browse over the tree for something like "Support for SMB 1.0/CIFS file sharing". These features are no longer part of newer Windows 10 builds - tick "SMB 1.0/CIFS-Client", and untick [in case it is] "SMB 1.0/CIFS Automatic removal". Apply and reboot the Windows 10 system.

This does - among the SMB 1.0 transport protocol also add the NetBIOS discovery and name resolution so the router can be recognized (instead of manually typing the \\192.168.0.1 for example).