Various theories have been proposed regarding the Sarah who married Samuel Magruder. While this has been disproven, The most prevalent legend regarding this Sarah is that she was Sarah Beall, daughter of Ninian.
This Sarah Beall and her family may or may not have actually existed and are the stuff of legends.
There are a number of theories as to who this Sarah is:
Sarah was born 1658,  was son of James, born say 1634, of Largo, Fife Co, Scotland.
Sarah was the daughter of Col. Ninian Beall. Note that Ninian Beall's father James was b. 1600. No major authors give credence to this. If Sarah were daughter of Ninian, then in the marriage of Elizabeth Magruder and Ninian Jr one would have the marriage of Sarah's daughter to Sarah's younger brother, a most unlikely combination!
Sarah was the sister of Mary Bell, who married John Pottenger That seemed to be corroborated by the fact that a Sarah Bell and a Mary Bell were transported into Maryland (although it seems most unlikely that they were sisters, since they arrived in Maryland 15 years apart. Brice Clagett (firstname.lastname@example.org believes there were no Beall Sisters: I suspect the "Beall sisters" myth for the wives of Samuel Magruder and John Pottenger originated in some one's speculation that Samual Magruder's wife Sarah must have been a Beall because the Beall and Magruder families were neighbors in Maryland and of Scottish origin, and that Samuel Magruder named a son Ninean Magruder. Then, I suspect, it was deduced that John Pottenger's wife must have been Sarah Beall's sister because he was Samuel Magruder's "brother."
Immigrated to Maryland in 1690 with Rev. Nathaniel Taylor. (FMMB ). Transported to MD 1675 (Immigration records, Brice Clagett) while Mary transported 1660. FMMB has arrival 1690 with Rev. Nathaniel Taylor. However, Brice Clagett (email@example.com) believes it unlikely that Mary and Sarah were sisters, or that Sarah was the wife of Samuel Magruder. The idea that Samuel Magruder's wife Sarah was nee Beall probably originated with Samuel's having named a son Ninean Magruder, but that falls far short of proof. (Ninean is a not-uncommon Scots saint's name; Col. Ninean Beall was the most prominent Scots settler in Maryland and a close neighbor, and presumably friend, of the Magruders.)
1686 Sarah Beall married Samuel Magruder, b. 1654
From the merged file: Spouse: Sarah Beall. Born: 1669 in Largo, Ligensheim, Fifeshire, Scotland, and died 9 MAY 1734 in Prince George's Co., MD. She was the daughter of Ninian BEALL and Ruth MOORE.
From the merged file: Due to a fire of Calvert county records there is no evidence that her father was Ninian Beall. If not, she was a relative. Ninian Beall's will mentions grandsons that are Sarah's sons. I see that Ninian is Sarah's father. He does have a dau. listed as Sarah. In this span of time there are three different Beals that married their cousins and combined with the Magruders. They are all from Scotland. Joshua and Josiah Beall are the same person.
The Legend of Sarah Beall
Sarah Beall's name exists primarily on popular genealogies.
Sarah Beall is said to have been born in 1669 in Largo, Fife, Scotland. 
Sarah Beall is sometimes said to be the daughter of Ninian Beall, but she is not listed in his will, and subsequent relationships between Sarah and Ninian's descendants would be incestuous if she were his daughter.
To son Geo. and hrs., 480 A., “Rock of Dumbarton,” on Rock Creek, and personalty.
To son Charles and hrs., 1,000 A., “Dunn Back,” on Wattes Creek, s. side Gr. Choptank, and personalty.
To granddau. Mary Beall and hrs. (dau. of son Ninian, deceased), after payment of legacies, ½ of personal est., also that part of “Bacon Hall” lying on s. side of road to “Mt. Calvert,” and to have her share at marriage.
To grandson Samuel Beall, ex., and hrs., water mill on Collington Branch and remaining part of “Bacon Hall,” providing that at 21 yrs. he makes over to afsd. Mary Beall a tract of land called “Sams Beginning” on s. side of sd. road to “Mt. Calvert.” Shd. he die before he be of age to convey land afsd., then the entire tract of “Bacon Hall” is bequeathed to sd. granddau. Mary Beall.
To son-in-law Andrew Hambleton, personalty.
To son-in-law Joseph Belt and hrs., 245 A., “Good Luck,” he to allow to heirs of testator 4,000 lbs. tobacco.
Two grandchildren of deceased son Ninian to be cared for and educated.
Test. directs that a tract of 400 A., “The Recovery,” in the Freshes of Patuxent R., at head of Weston Branch, and adj. land bequeathed to Joseph Belt, be sold for payment of debts.
Sons Charles, Joseph Belt and Geo. to aid executor until he arrives at age of 21 yrs.
Test: John Busey, Rebecca Getward, Ed. Willet. 
Samuel and Sarah named a son William and had a grandson by their son Samuel, named William Mills Magruder.
Sarah who married Samuel Magruder could not be a daughter of Ninian Beall
Source text: It is well documented that Ninian Beall had a son Ninian Beall, Jr. who married Elizabeth Magruder, daughter of Samuel Magruder and wife Sarah. See the 1731 will of Samuel Magruder's widow Sarah where she named a daughter Elizabeth, wife of William Bell. Elizabeth had married first, Ninian Beall, Jr (son of Ninian and Ruth) and secondly William Beall. If Elizabeth Magruder's mother Sarah was a daughter of Ninian Beall, Sr., that would mean that her first husband Ninian Beall, Jr. (son of Ninian, Sr.) was her blood uncle. Marriages of that close a relation were forbidden then, just as now. There is NO evidence in original records to indicate that Sarah was a Beall, much less a daughter of Ninian. The "thought" that she was a Beall appears to originated from the fact that Samuel Magruder and wife Sarah had a son named Ninian. It was not uncommon to name family members after friends, church elders, leaders of the community, etc., of which Ninian Beall would have been all three to the Magruder family. Members wanting to join a lineage society in the early 1900s "created" [forged/falsified] this relationship so that they could join under Ninian Beall, and thousands of people have accepted this forgery as fact, and spread it through their records.
Reason attached to person: to show why Sarah cannot be a daughter of Ninian Beall
I would like to remove Sarah's existing parents, Ninian and Ruth (Moore) Beall, from this profile. Please let me know if this is okay. Based on the evidence Jack Day listed (in the profile itself) and Maryland and Virginia Colonials, which corroborates that Ninian Beall did not have a daughter named Sarah, I believe these parents should be removed.
Beall-201 and Beall-81 appear to represent the same person because: Both are married to the same Samuel Magruder. (Though there's apparently some debate about whether Sarah is actually a Beall; see the thread here: http://genforum.genealogy.com/magruder/messages/1126.html. Nonetheless, whether or not Sarah's a Beall, I suspect they're the same Sarah!)
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