You can detect the presence of back blocks from a failed flash operation (such as an Erase) or an invalid Write operation (discovered through an invalid Error Correction Code, or ECC).

如果 flash 操作（例如 Erase）失败，或者 Write 操作无效（通过无效的错误校正代码发现，Error Correction Code，ECC），那么说明出现了坏块。

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During pairing, red light and blue light flash alternatively. If pairing failed, blue light will flash once per 2 seconds.

配对时，红蓝灯交替闪烁。配对失败后，篮灯每2秒闪烁一次。

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When reminded of this, Toshiba admits that it did, in fact, invent flash memory but failed to capitalize on its initial lead.

FORBES: Magazine Article

Intel has failed to wring consistent profits from its flash memory business, which last year accounted for 6% of its sales.

FORBES: Otellini To Cash Some Of Intel's Chips?

But after an anxious 25 minute wait, the flash igniters set up around the leaking gas pipe failed to go off.

CNN: November 21, 2012 -- Updated 1203 GMT (2003 HKT)