In a note to clients Thursday, Morgan Stanley’s Katy Huberty rattled off several reasons to explain why retail sales of personal computers in the U.S. were down 8% year over year for the week of Nov. 20 to 26, as reported by the NPD Group.

• Hard drive shortages, the lingering effects of flooding in Thailand
• Price reductions in the two weeks before Thanksgiving, which encouraged early buying and cut into Black Friday sales
• Tablet cannibalization, especially from Amazon’s (AMZN) Kindle Fire and Apple’s (AAPL) iPad

“But,” Huberty concludes, “we believe the biggest factor is simply weaker end demand. The data so far in November suggest a steady sequential deceleration.”

There was, Huberty writes, one exception to that rule:

Apple was a relative bright spot during the week of Thanksgiving. Unit growth rebounded to 17% Y/Y from 7% in the prior week and 10% in the prior four weeks.

The MacBook Air, which uses solid state drives instead of those harder-to-get hard drives, now accounts for 30% of Apple’s notebook sales, according to NPD.