Good morning and welcome to the Seagate Technology Fiscal Second Quarter 2018 Financial Results Conference Call. My name is James and I will be your coordinator for today. At this time, all participants are in a listen-only mode. Following the prepared remarks, there will be a question-and-answer session. As a reminder, this conference is being recorded for replay purposes.At this time, I would like to turn the call over to Kate Scolnick, SVP, IR and Treasurer. Please proceed, Kate.
Thank you. Good morning, everyone, and welcome to today's call. Joining me today from Seagate's executive team are Dave Mosley, CEO; and Dave Morton, EVP and CFO. We've posted our earnings press release and detailed supplemental information for our December 2017 quarter on our Investor Relations section of our site at seagate.com.
During today's call, we will review the highlights for the December quarter, provide the company's outlook for the March quarter, and then open the call for questions. We are planning for the call today to go approximately half an hour, and we will do our best to accommodate your questions following our prepared remarks as time permits. We've received a lot of interest in our next strategic update and we will be finalizing a date shortly, most likely in the early fall time frame. In addition, we would like to note that our March quarter quiet period will begin on March 26.
(Forward-Looking Cautionary Statements) I would now like to turn the call over to Dave Mosley. Please go ahead, Dave.
Thanks Kate. Good morning, everyone, and thanks for joining us today. For today's earnings call, I will cover the high level trends we're seeing in the business. Our CFO, Dave Morton, will then discuss certain financial highlights, and I will close the call with our outlook for the March quarter. Beginning with our operational results for the December quarter, Seagate achieved revenues of $2.9 billion, up 11% sequentially and up 1% year-over-year, GAAP gross margins of 30.1% and a net income of $159 million.
GAAP diluted earnings per share were $0.55 including a one-time provision for income taxes of $208 million related to tax reform. On a non-GAAP basis, Seagate achieved gross margins of 30.4%, net income of $431 million, up 5% year-over-year, and diluted earnings per share of $1.48, up 7% year-over-year. HDD exabyte shipments for the December quarter were a record 87.5 exabytes, up 28% year-over-year.
The average capacity per drive across the HDD portfolio was also a record 2.2 terabytes per drive, up 29% year-over-year, and the average selling price per unit was $68, up 3% year-over-year. GAAP operating expenses were $444 million, down 15% year-over-year, and non-GAAP operating expenses were $390 million, also down 15% year-over-year. Cash flow from operations for the quarter was $850 million, up 30% year-over-year, and free cash flow was $773 million, up 38% year-over-year.
Achieving revenue and profitability growth and significant cash flow generation in the December quarter reflects Seagate's strong execution and the competitiveness of our storage portfolio, particularly