I admit to changing my mind a few times about this question. The candidates are the Battle of Preston, 1648; the Battle of Dunbar, 1650; and the Battle of Worcester, 1651. At the moment (June 2021) I am leaning toward Worcester. Here are the arguments.

In an essay published in The Yearbook of the American Clan Gregor Society, Vol. LXIII (1979), Charles Kurz wrote:

Tradition is that Alexander McGruder served in the Scots Army of King Charles II, was captured after the Battle of Worcester on 3 September 1651, and was later transported to Maryland. In the context of Scottish and Maryland history, students of Alexander Magruder’s life have difficulty accepting the tradition that he was captured after ‘the Battle of Worcester’. The time span from the purported 3 September 1651 capture is difficult to reconcile with a time for sentencing as a rebel, a 3- to 6-month voyage to Maryland, a period of indenture, and two land assignments of 50 acres each on completion of indenture–all within 26 months–by 19 November 1653…According to T[homas] G[arland]. Magruder, Jr, it is very possible that Alexander McGruder could have een captured at the Battle of Preston on 17 August 1648. (68)

I have argued in the past that it’s most likely Alexander was captured at Dunbar on 3 September 1650–two years after Preston and exactly one year before the Battle of Worcester. The defeat at Dunbar was huge and Scots prisoners many. Accounts I have read differ in some details, but agree that hundreds, if not thousands died on a forced march to Newcastle, even as their captors debated what to do with them. In true Puritan fashion, a committee was appointed–which quickly determined that those healthy enough to work might be sent to the coal and salt mines. As early as September 16th, the committee was receiving petitions from entrepreneurs who wanted to profit by transporting prisoners to the colonies as indentured labor. Thus, these captives from Dunbar became the first prisoners from an internal war to be transported–barbadoed in the slang of the day–a decision that was unprecedented and notably scandalous. From these accounts, I think it unlikely that Alexander, or anyone else, was transported to the colonies after the Battle of Preston two years earlier. 

However, prisoners from Worcester, a year after Dunbar, were quickly marched south to London, thousands dying on the way, to be prepared for transportation to the colonies. A strong argument for believing that Alexander was among them arises from the way regiments were raised and commanded. There was no “Scots army,” as Kurz puts it; regiments were raised locally and personally by titled and landed gentry, from among their tenants and allies. Lord Madderty’s brother commanded a regiment at Worcester that was overcome, with many (including Lord Madderty) captured–a perfect fit for Alexander, whose father was Lord Madderty’s chamberlain. Given his father’s status, Alexander would have served as an officer in such a regiment. (I have seen American accounts assigning him a rank but this is, pardon the pun, rank speculation.)

For Scots prisoners shipped to New England in these years, many records survive, including their names, the ships they arrived on, and where they were sent to work. Unluckily for us, no such records survive from Maryland or Virginia.

In the Yearbook of the American Clan Gregor Society for 1952, Herbert T. Magruder published “Some Notes on Circumstances Surrounding the Transporting of Alexander Mac Gruether as a Prisoner of War; and His Landing in Maryland in 1652.” He writes that

[r]esearch by William H. Gaines, Jr., an editor of Virginia Cavalcade, a publication of the Virginia State Library, has brought to light a record of the fact that a fleet of two warships, the JOHN and the GUINEA, and  several armed merchantmen was fitted out by order of the Parliament in the summer of 1651. The ships sailed from England in the fall of that year. Among the passengers were six hundred Scottish prisoners of war, who were being sent to do penance in the colonies. The voyage was by way of Barbadoes, which was a center of Royalist resistance as was the Virginia colony. The weather at the season may also have been a factor in plotting the course; for the  John was lost at sea; and the Guinea lagged behind. The merchantmen therefore arrived first off the Virginia capes in January 1652, and anchored offshore to await their armed escort. At length Governor William Berkeley and his Council were brought to terms of submission by the Parliamentary Commissioners; and the fleet sailed up the Chesapeake to bring the Maryland colonists into line.

It is certainly probable that Alexander Mac Gruether was one of the six hundred Scots on board those transports[.]

If Alexander was captured at Worcester, the timeline in this account could explain the rapidity with which he reached Maryland. Historians say that Cromwell disliked the costs of housing and feeding prisoners, and moved rapidly to dispose of them. Chris Gerrard, research director of the Scots Prisoners Project at Durham, has given me his opinion that Worcester is entirely feasible as the place Alexander McGruder was captured. There is, however, nothing to connect Alexander or his fellow Drummond retainers to the burials at Durham.

That said, the Scottish Soldiers at Durham project is well worth following, as they study (and try to identify) the remains of prisoners from the battle of Dunbar, imprisoned and later buried at Durham Cathedral. A victim of the Reformation, the building was empty in 1650 and had not been used for worship for several years. Mass burials of the 1,600 men who died there (out of 3,000 imprisoned) were discovered in 2013 in a routine construction project. Read this wonderfully informative interview with Christopher Gerrard, leader of the research team, at the Council for European Studies.

Some further background…Maryland had been plundered by Puritan freebooters as early as 1645, when its Catholic governors were arrested and sent to London in chains. Those with the resources to flee had hightailed it back to England, or at least to Virginia, leaving the colony to the lawless rule of Puritan-backed thugs for the next two years. By the time the Calverts returned in 1648 (bringing with them a Protestant governor to appease their enemies) St. Mary’s City held fewer men than had founded the colony in 1631, all other survivors having scattered into the woods and swamps for self-protection. It was in this atmosphere of desperation, as the Calverts struggled to keep hold of the remnants of their colony, that the famous Act Concerning Religion–so often touted as the first American law to guarantee freedom of conscience–was passed by the hastily reorganized Assembly. It failed in its first purpose, however–to convince the Puritans to keep their hands off Maryland. In the following year came the Regicide and Cromwellian rule, followed in due course by the battles of Dunbar and Worcester, and the final submission of Virginia and Maryland to the forces of Cromwell’s Parliament.

This was the Maryland to which Alexander was delivered.

Several sources give the number of prisoners still on board when the Guinea reached Maryland at 150. Wherever he was captured, and however long the hell of his ordeal, it is probable that Alexander Magruder was among them.