To continue my previous post…
(I wrote this already, but my post got lost, so I’m typing it in Notepad first… yeah, that’s handy).
I contacted SanDisk support about my issue. They assured that the drives were tested for safety and compliance, but they agreed to replace them by another equivalent model. The RMA process took ~2 weeks and in the end I got my UltraFit drives replaced by 2x full size “SanDisk Ultra USB3 64GB”. They seem to work fine without issues - there is no speed drop and the drive remains barely warm during large file transfers (~10GB).
Despite I was not happy with the product (UltraFit), I admit that SanDisk support was helpful and resolved my issue by replacing the drives with another equivalent model.
PS: certain people with a “Guru” badge here claim that the issues are caused by filesystems or some other mythological reasons. That’s wrong. A storage device should not care what filesystem or OS is used as it should be designed to work correctly under any load providing the ambient environment is within its limits. Please do not confuse people with your silly myths.