This post more to document and discuss, learn than anything else. Background: the usb stick had been 'painted' with many layers of nail polish by an artist who wanted to make it into a present. As such i felt it was worth the effort and an interesting experience.
When asked to look into why an USB stick could not be used anymore i ran into a number of bizarre events. Eventually resorting to testdisk and running Analyse my suspicion was confirmed this stick was somehow damaged.
Before: The original pictures are on a portable usb hdd drive attached to a mac, the usb stick is as purchased. Both on an Apple OSX machine and a MS Windows machine it was found the stick would not accept the pictures as before. Every app i tried said the stick was a 64GB FAT32 filesystem. I assume there is where things went wrong. I tihnk to have seen this once before where an Apple screws up the filesystem since to OSX FAT32 cannot hold such a large size volume (64GB)
Initial actions performed were repartitioning, on each occassion this resulted in a success-fail condition with recurring i/o error notification. Initally on OSX (High Sierra) later on Linux. Multiple repartitioning attempts on both OSX and Linux failed. Somehow showing historical attempts for different filesystems present depending on what tool was used. This could be one tool finding ntfs, another ext4, another hpfs. Partitions could also "loose" information after a reboot.
Eventually, after many partitioning attempts on a Linux system (gdis, cfdisk, parted, testdisk) I decided the have testdisk wipe and create an MBR. Bizarre to find after reboot it showed a GPT table as was attempted before. Now using testdisk to create a partition was considered, which again worked, then failed. More attempts, rebooting etc to no avail.
Either way, eventually, i ran analyse again. Somehow, now it stopped at 75% and testdisk was unresponsive. Next i decided to use gdisk to create the partition from 0%-70% and found it worked. No more issues. No i/o errors or anything. Mutiple tests later, still stable at +40GB out of 64GB.

Summary: Now i felt like i eventually had a success but not a solid approach. At one point i suspected some malware on the USB Flash drive, maybe even in the firmware. Once i managed to reduce the volume size and it worked stable i kind of let go of that idea.
Question is obviously what program i could have used to find this issue.
It is probaly against the grain of the author's intent but partitioning could be extended as a separate functionality imho.
Maybe also some speedy analytics to simply inspect and report on garbled volume structures.
In the end, if it were not for the analysis halting at 75% i would not have guessed a functional volume size that easily.
I'd love to hear from you.