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Ahh thanks makes sense 

I was copying and verifying a 4tb disk to another one 

Very large files and found after hours of copying some of the large files had mismatch md5

I could have known this problem hours earlier if it had option to verify after :-)

Turned out it was a flakey external usb disk enclosure

i jumped up and down on it till i got blister after that and bought new enclosures

Thanks all