Updates to External Pen Drive Blocked

I backup, or to be more precise update, files (mainly spreadsheets) to an external usb pen drive connected to my BT router. Recently I have been blocked from doing this with a message stating that I need permission to do this. I would have included A copy of this message but I am unable to include an attachment for some reason. My computer is a Dell Optiplex 7010 running W10 version 22H2 (OS Build 19045.3324) and is connected to the router via an ethernet cable
This permission requirement also occurs if I try to update the same files from my laptop which is connected wirelessly to the router. The usb drive has a capacity of 256GB and is nowhere near full.
I am at a loss as to why this has occurred and how to fix it, can anyone help?

What model BT Home Hub is it?

Are you accessing the pendrive using \\192.168.1.254\usb1 from a File Explorer window?

Is it the pendrive itself or the files within that you are getting the 'permissions' notification?

Can you try again to get a screenshot of the notification?

Hi Dalchina, I tried saving a new file to the pen drive but I got the same result - I needed to get pemission. Where would I find my network permissions - somewhere in settings? Yes there was an update recently which incuded a net update.
However I have just tried removing and re-inserting the pen drive and I have been able to update several files! What only time will tell is how long this will last.

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Hi RickC, My BT hub is as far as I know a "Smart Hub". Yes I access the pen drive using desktop shortcuts but essentially the way you suggest. and it is at the point of updating the file that the permission is being requested. I've tried again to attach the permision widow but to no avail.
I have had some success however, see my reply to Dalchina above

Hi Dalchina, I found that the next time I started my pc that again I was blocked from saving to the pen drive, however I have now done a restart (I know this can have a different effect from just shutting down and starting) And saving to the pen drive is still working. I have tried this several times successfully. If I continue to experience trouble I'll investigate the references you have detailed above, thanks again for your help

Hi Dalchina, So far so good, restarting my pc seems to have been effective although yesterday the saving process froze. It's working ok again totay however. So restarting the pc or sometimes removing and reinsering the pen drive work so I'll live with that. As you indicate it may have been a Microsoft update that caused the problem and maybe if there are sufficient complaints they may rectify it in the future. Thanks for all your advice.