I have found that the Sandisk USB disks will fail, especially under low voltage conditions (underpowered adapter, for example).
Alistair gave me instructions when mine failed.
- Power off the RIPE Atlas Probe
- Remove the USB stick
- Power on the tp-link and connect it to the network
- Wait for 5 minutes
- Insert the USB stick while the RIPE Atlas Probe is powered.
If that doesn't work, try a new 4GB USB drive instead.
SanDisk was good about replacing the failed USB drive, but I think I'd use a different brand than can tolerate lower power.