Since my post above on 16 May 2020 my TS251D NAS has been using 18.104.22.1686 firmware with the issues mentioned, but with external USB drived (for back-uo) being recognised.
I recently upgraded the firmware to 22.214.171.1244 to keep security etc matters up-to date, and to get rid of the reminders to upgrade.
Lo & behold, back-up to USB drives no longer successful.
Might I gather that this has not yet been remedied, and that another firmware downgrade to 126.96.36.1996 is required? Or am I doing something wrong?
Meantime, I will downgrade to try to get my backups running again.
If backups still fail I'll have to look at another solution.
Backup, Restore, Netbak Replicator, Cloud Storage Services