General McClellan, in "McClellan's Own Story" he notes that his army was pressing on Lee with the dual purpose of catching Lee's army in battle before he could abandon Maryland at or about Williamsport, and relieving Harper's Ferry, From a letter of the evening of Sept. 12th:
Then on the evening of the 13th he received the copy of Lee's order of Sept. 9. Mac says the receipt of the "Lost Order" the relief of Harper's Ferry became secondary...as he believed he had a chance to actually cut Lee off from the river at or about Boonsboro, so the army's forward movement was directed principally thither: p. 572:
By the night of the 14th, per McClellan, Franklin's corps was within six miles of Harper's Ferry...
But Franklin was directed to Rohrersville, to straddle the road to Harper's Ferry, BUT to WATCH out for attack from the presumed (per Lee's Lost order) Confederate army at Boonsboro rather than move direct to the relief of Harper's Ferry. Franklin to attempt to "cut off" any retreat by Lee from Boonsboro toward the river at Sharpsburg...
On the 15th Franklin couldn't cut Lee off from withdrawing to Sharpsburg, as he was already concentrating there about the time Mac's orders above were cut...
From Longstreet:
And Harper's Ferry capitulated to Jackson that day.
Franklin reported to Mac at 8:15 A.M. that per his 1 A.M. orders, he was concentrating to attack Rohrersville...