Please insert a disk into Removable Disk (N:).

Do you have a built-in card reader?
Does Explorer show N: under My Computer?

If you're absolutely sure N: shouldn't be there....

Right-click the start button.  Select 'Disk Management'.

When the window opens, check for N: in the list of drives/devices.  if present, right-click and select 'delete volume'.

I suspect nothing will show in there though as it's asking for a disk that is NOT present.  In that case, check in Device Manager for disks.  You may have one listed with a faulty driver.

After that, I would start considering SSD memory card slots if this is a laptop.  Is it?

Computer Management>Disk Management.  Right-Click the drive in Question and change the drive letter

Also, run MSConfig and for testing purposes, untick 'load startup items'.  Restart.  If the issue is cleared, then systematically go through the Start Up Group items on the Startup' tab, re-enabling one at a time until you find the culprit.  Unless one sticks out like a sore thumb, in which case go for that 1st.

Drive N: in Windows 7 was for the Microsoft Virtual program.  Look in add/remove programs and uninstall if you are not using

Thanks all.

It does show up in File Explorer, does this screenshot help at all?

General properties: 0 space used; 0 space free.

N does show in Disk Management, as 2nd screenshot shows.
I do have two other USB slots in use - the wired printer and wired keyboard.

Changed drive letter to P, same problem. Only with P!

I can't find Microsoft Virtual program in my list of programs.

Have you recently used one of your card readers

Good point.  I have never used the one in the printer (didn't know there was one!) and the Sandisk drive is an internal HDD.  But, I have used a USB card reader quite a lot and probably didn't eject it the proper way at some stage.

Ahh - you caught me out editing my last post when I worked out the SanDisk was an SSD :)
See if the error still appears if you restart with the printer unplugged
If it does then yes Windows has a temp file path to/on the removable card on the now missing card reader - I should have been more circumspect in my first post "do you now or have you ever owned a removable card reader ..." :)

Plug the card reader back in put a card in N/P and that should help you clear it.

Yeah fooled me too, SanDisk have diversified.

Have restarted with the printer unplugged.  OK so far........

Yup, guess it's fixed.  Printer back in and all OK.

Cheers

Thanks!