Let's say I'm backing up my sim (all 50 gigs of it

I have the correct chipset drivers installed, the drives SMART status is normal and they test fine, and Memtest runs without errors.
I have found that I can copy the whole 50GB folder in one lump, if I do so in "safe mode", which made me think it was a driver issue, but again, I have double, double checked all of them and everything seems right.
Could it be a PSU issue, and why does the process work in safe mode?