Western Digital Corp. is seeking to raise $3.09 billion from the sale of a stake in Sandisk Corp., after the digital storage company spun off its flash memory unit nearly a full year ago.The shares are being offered for $535 to $555 each, according to people familiar with the matter. The discount represents as much as a 9.4% discount to Sandisk’s $590.59 share price at Tuesday’s close. The offering is well oversubscribed following a confidential marketing effort, the people said, asking not to b...