I'm back to not being able to make head or tail of it
From an install of regular 7.4 on C I can see images on C and H, but not on D.
From an install of portable 7.4 on D I can see images on D but not on C or H.
From an install of portable 7.4 on H I can see images on C and H, but not on D.
From an install of portable 6.29 on C I can see images on C, H, and D.
From an install of portable 6.29 on D I can see images on C, H, and D.
From an install of portable 6.29 on H I can see images on C, H, and D.
That suggests it's not a flaky drive or a permissions issue.
By 'see the images' I mean see image files in Book Details that were embedded in Comments via the Link tool File button in MDE.
I'll have another look at it tomorrow
BR