>2.43×10^-15 electrons
I believe TFA reads 2.43×10^-15 kg, not electrons. Unless SSDs are creating new and exciting physics, one can't have less than one electron, as it's an elementary particle.
Neutrinos weight far less than electrons (but while NAND flash involves super weird physics it's not that weird)
10^–15 is not a negative number, just a small one. https://www.wolframalpha.com/input?i=10%5E-15+
Well you could have a virtual particle whose mass could be time-averaged.