I want to be clear that you understand that "7000" is not the problem, it's "7000 in 30 minutes" or as in other versions, 10 minutes or less than an hour. Or sometimes even 8 minutes. There is simply no way to convert the casualty data into such a small time window.
Neither Catton ("... on June 3 the Federals lost rather more than 7,000 men, most of them in the first two hours of fighting." _Grant Takes Command_ page 267) nor MacPherson ("The Yankees suffered 7,000 casualties this day." _Battle Cry of Freedom_ page 735) repeat the "30 minutes."
Rhea's position is that breaking down the
official casualty number (12,788) for the Cold Harbor Campaign into the various actions that occurred is "elusive." His analysis of the casualties covers about 30 pages, but his best approximation (pg 392) is that Federal losses were about 5,000 May 28- June 2 and 6,000 on ALL of June 3, not just the failed assault early in the am. The remaining casualties occurred through June 15th, the end of the reporting period.
So Catton's and MacPherson's figures are higher than Rhea's, by about 15%. However, having read all three (I've read Trudeau as well, but don't have it at hand) none repeat the "30 minutes" and their actual estimates look to all of June 3 and not just the AM assault.
Rhea spends 30 pages on the casualty numbers, Catton and MacPherson, not much more than a sentence. I don't doubt Rhea put forth a much greater effort digging through the reports and that his number is most likely the better one. It's certainly the one best supported by the available evidence.
So Rhea has a rather minor difference from Catton and MacPherson, but none of these 3 support the statement "At Cold Harbor there were 7000 casualties in about a half hour."