Earnings previews from Western Digital (WDC) - Get Western Digital Corporation Report and Silicon Motion Technology (SIMO) - Get Silicon Motion Technology Corporation Sponsored ADR Report have created optimism for memory chip companies, though the outlook may not be universally rosy.

Western Digital said after the close Wednesday that it expects fourth-quarter revenues of $3.46 billion, up from a range of $3.35 billion to $3.45 billion, but did not say whether the improvements arose from hard drives or NAND flash sales.

"NAND pricing started to improve in June. That could have resulted in better average selling prices for SanDisk," says Cowen analyst Karl Ackerman.

Western Digital gained $2.20, or 4.8%, to $47.66 on Thursday, and was up another 1.3% in Friday morning trading.

Micron (MU) - Get Micron Technology, Inc. (MU) Report rose 47 cents, or 4%, to $12.20 on Thursday. The June increase hasn't yet impacted Micron, Ackerman noted, because its most recent quarter ended in May.

"You probably saw some upside from lower-end PC hard drive shipments," Ackerman added, which would add revenue but not gross margins.

Hard drive maker Seagate (STX) - Get Seagate Technology PLC Report rose 0.9% on Thursday to $23.73, after being up as much as 3.5% earlier in the day. On Friday morning, it was trading up 0.8%.

"I don't really think you can assume a positive pre-announcement from [Western Digital] is necessarily indicative of a stronger-than-expected quarter from Seagate," Ackerman said.

The company has taken steps like increasing the average selling prices and shifting away from lower-end PCs.

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"Thus, while we saw better notebook PC [original design manufacturer] shipments through May it may not necessarily benefit [Seagate] all that much," Ackerman said.

Morningstar analyst Timothy Feeney said that Western Digital will benefit from the flash offerings it added through the $16 billion acquisition of flash drive maker SanDisk, which closed in May. The move will offset risk from flash drives replacing Western Digital's hard drives.

"Their strategy seems to be more plausible than Seagate's, just because Seagate is going to stick to hard drives," Feeney said.

Though sales of hard drives have been rocky, Freeney said, they still have a place in corporate data centers. "They are still from a price performance standpoint the best out there right now, and data centers are going to rely on hard drives for storing persistent, raw data," he said.

The analyst questioned the sustainability of Seagate's dividend yield of about 10%, given its strategy. Seagate declined to comment on the dividend yield.

Meanwhile, on Thursday morning Silicon Motion, which makes circuits for NAND flash and mobile devices, gave a favorable preview. The company expects revenue to grow 23% to 25% in the second quarter, a significant upward bounce from its prior growth estimate of 5% to 10%.

Silicon Motion rose $2.45, or more than 5%, to $50.39, on Thursday and was up another 2.9% in Friday morning trading.

Cowen's Ackerman noted that Silicon Motion's news could be "slightly positive" for Marvel Technology's (MRVL) - Get Marvell Technology Group Ltd. Report storage business.

Marvel shares gained 33 cents, or 3.5%, to $9.77 on Thursday.