Firefox gives no disk in drive error message

Ever since yesterday, Firefox was giving me the message "There is no disk in the drive. Please insert a disk in '\Device\Harddisk1\DR6'" (see pic) whenever I tried to save a file in firefox (or in some cases open a file from the downloaded files dialog box.
I think this started when I removed my SD card from the card reader whilst Windows was still trying to write to it (oops!) as it complained (despite having done it a couple of secs after the app I was using (explorer) had updated its screen AND write caching is disabled on the drive)
A reboot of the box didn't fix the problem and a search in google had lots of people with similar error messages but from different applications but no fix. However I found that by inserting the card back into the card reader and then using the stop hardware function in the notification area the problem, so far, seems to have gone away.