Repair bootable USB stick

[Jon Magee <mageejp@riseup.net>]

I recently helped a friend install Mint on her computer, and I made a 
bootable USB stick using their .iso and dd. It's a 16Gb drive, and the 
.iso was 3.9Gb. I now have the problem that I can only format 3.9Gb of 
the drive.

I'm on Debian 10.2 with KDE. GNOME Disks utility recognizes the device 
as "16 Gb Thumb Drive / Sandisk Cruzer Glide," and it shows three 
partitions: a 2Gb ISO9660 for Mint, a 2.5Mb FAT, and 14Gb free space. It 
also shows partitioning as "Master boot record." But when I press "-" on 
the Mint partition I get the following error:

"Error deleting partition /dev/sdc1: Failed to read partition table on 
device '/dev/sdc/ (/dev/sdc: unrecognised disk label) 
(udisks-error-quark, 0)"

When I try to format the free space, I get this error:

"Don't know how to create partitions this partition table of type 
'(null)' (udisks-error-quark, 0)"

Originally I got an error from GNOME Disks that the device had no 
partition table. I tried the KDE partition editor to no avail. That app 
made a new MS-DOS partition table for the device, but it believed it was 
only 3.9G. Making that partition table has not changed the results in 
Disks, other than to change it from the no partition table error to the 
unrecognized disk label error.

Can anyone offer any help?

Thanks,
jon