brand new WD drive has odd errors

it is a separate, external drive
I ran it thru the very long 2 day test and it passed. Data lifeguard by MD
files vanish.
and now this.. i will make the numbers even for this post.
have a file on another drive that has 2 files inside.
one 250M and the other is 500M. so the main file says 750M
sure does on all other drives I tested-
except this one - it reads 500M even though the two files in are correct at 250M and 500M !
deleted it and copied it again- same error.
this is new to me.
any ideas?
I am backing up files and need the sizes to be correct.
thanks all

Important. these files are on an HP computer- not the dell on the previous posting.
it is a few years old and has a disc drive- wish i had bought 2!

i do not know what you mean.
we are not all gurus!
thanks

Could it be a folder that contains one 250mb file plus a 500mb file ?

the same folder that has 2 folders in it
is a different size when copied to one drive that the others,
it seems to only record the larger of the 2 files, although both files ARE within the main file.
it does not make sense to me either, that is why i asked!
thanks guys

folder A contains 2 files B and C
File B is 500M, file C is 250M

Folder A on one of the external drives is 500M- even though files B and C are there and functioning. they are videos.
i have contacted WD

Is this any clearer?

it is so unbelievable, i will - as suggested - have to post a screen shot
thanks all

i tend not to use cmd line as i seldom know what i am doing!

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does this help?
thanks