Making backup to USB Stick

[Helvetica]

On the advice of a very helpful user here, I need to make an image backup before making an alteration to my settings file.
I went to the BlackHole Plugins menu and clicked on "Full Back-up to USB"
I have now tried a total of 6 different sticks of different content size and of different makes, formatted to FAT and then (next attempts) to NTFS
I also tried one formatted using the BH USB Formatting function.
I have added empty files called backupstick and backupstick.txt, I have tried all these in all combinations in both USB slots and every time I get the message
"there is no valid USB stick found".... and that if one of the above named files is present on the stick (which they ARE), then it will do a backup...

What is going wrong ? I am truly close to tears.
The USB slot at the front definitely works because last time my VU+ Duo crashed, my son-in-law used a USB stick in it to put my Black Hole 3.1.3C image on it.
But the BH Back-up function doesn't want to recognise any of the sticks I try.

When I try Main Menu > Backup Menu > Blackhole Full Backup, I can't change the "save to" location from the harddisk to USB, and when I let it save to harddisk, it tells me I don't have enough room on my harddisk (which is 1 terabyte!) and the same applies to the Personal Backup function.