@agw_01 is right on the money, 48c for drives isn't terrible (I've seen servers in businesses with drive temperatures that have gone to the low 50s) however it entirely depends on the drives used as to what their recommended temperature window is (However most is normally under 60c)
What I would say is like most things, a higher temperature (Within parameters) will result in faster degradation of the hardware, maybe not by a huge amount but it will still have a negative impact in the long run so I would be doing what you can to try and drop the temps be that increasing the fan profile or checking the unit to see that it's not got too dusty from being in the loft.