Samsung Galaxy S5 — False reporting that external fat32 hard is corrupt

Hi folks, I'm not a newbie to android nor to Samsung phones and I guess I'm a geek with Windows OS. My S5 SM- G900F which is on stock Android M 6.0.1 has started to report my external Fat32 2gb hard drive as corrupt/Unknown file system when I know it's not as both my S5-G900A and A10-70F tablet read it perfectly. This is the third time this had happen now. It happen twice before ,once on kitkat and then again on Lollipop. I have done a complete diagnostics test on the hard drive on my win7 64 bit laptop using specialised software ( I work as an IT/network consultant part-time). I have tried to figure out why this should happen or what is causing it but I am at a loss. Both S5s have the same apps although the 900A is stuck on AT&T version of Android. Anyone else have this issue and anyone got an idea what's going on. I've done rooting in past and also created my own custom version of Android on my old S3 a few years back so know my way around Android OS. Thanks in advance.