Further to my post on 9 May 2014 we did visit the area and found the site of the mill, Croftbeg and Druiminlochan, based the old maps that we found at Scotland’s Places
Yes the Mill cottage referred to by AlanBoyd is still there. Just downstream of it on the same side you can see the remains of the mill race which would have turned a wheel to power the mill. The mill would have been a timber structure and some flood must have washed it away at some point.
Pretty much opposite there is a small cottage named Feshieside Croft, which I take to be where Croftbeg stood in 1851 when Alexander and Betsey Grant and their family were. The old maps show there was a footbridge across the Feshie to the mill
In 1841 they were at Druiminlochan, which these days is a beautifully renovated house higher up the hill on the main road, overlooking Lochan Gael.
My wife is descended from Betsey Grant b1805 who was daughter of Murdoch Grant. I now suspect, based on her DNA matches to descendants of John the sawmiller, that Murdoch may have been his brother. There is a Murdoch Grant of Strathspey who enlisted in the Gordon Highlanders in 1805, so my working theory is that Murdoch Grant’s wife (Betsey’s mother), died, possibly in childbirth, and Betsey was left with the family of John the sawmiller.
My Grant family tree on Ancestry is here:
https://www.ancestry.co.uk/family-tree/person/tree/120695841/person/332148132918