Didn't know until now that USB sticks could become useless altogether after years on inactivity. Is this also applicable to the sticks which are produced in the last few years? I have yet to back up my wallet in any USBs so this is a great piece of advice that I just came across with, for future reference.
Magnetism wear off after a while and the media it is stored on also deteriorate after a few years. If you plug it in and leave it for a while, the magnetism will increase and it will last longer. Just use a offline computer when you doing this, or your data stored on the device could become infected or even collected by a hacker.
In any way, this is how it was explained to me. I hope the guy was not pulling this out of his ass. ^smile^
Data in USBs and SSDs are not stored using magnetism, they are rather, stored in tiny computer chips. This is also the main reason why most of them are very durable. However, a potential problem about USB and SSD is the limited read and write life which isn't an issue with low usage. The quality of the SSD or USB is important as it is directly related to the data retention of the media.