I am very happy to hear that things worked out for you. 
But I don’t understand how sandisk could prevent an app writing to the boot sector.
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Sandisk is a drive, composed of bits, bytes and clusters. No software.
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The only difference an app writing to the drive may encounter is whether Windows considers it to be removable or not.
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The boot sector is simply a cluster with a low number, though important for booting not particularly different than other clusters.
I would love to see an screen print of the error msg you encountered.
Glad things worked out for you and thank you for posting your update…
