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When the APM value of the drive is set to 254 or 255 (head parking is disabled) using either hdparm or HDDScan utilities, my HGST 7K1000 HTS721010A9E630 hard drive is occasionally making normal quiet activity sounds, but in periodic short bursts lasting (hard to say...) from 0.1 to 0.5 seconds and occuring every 1-3 seconds. The problem is thoroughly described in another question here. If head parking is enabled (APM value is 128), the drive clicks after it's been accessed, and no further sounds are heard until the next access to the drive.

I don't know whether changing the APM value using Hitachi Feature Tool will give the same problem, but I'm not willing to use it yet, until I figure out whether using it might void the warranty. I asked a question about the warranty both here at Super User and at WD Community website. No answers yet.

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After several hours of experimentation, thinking and reading I came up with an answer. I have a rather crazy, but plausible assumption that the makers of HGST 7K1000 HDD were a bit lazy, and instead of implementing the disabling of head parking, they made the drive to make some activity (read something, I guess) in order to keep it active, so that the timer of inactivity never reaches a certain point, thus the drive is making activity sounds about each second. So, in other words, my guess is that it was easier for them to make a timer to create activity each second, rather than to change the timer which parks the heads.

Also there is a less crazy explanation, but still a bit strange: setting APM value to 254 or disabling the APM puts the drive into Active Idle mode, and it is supposedly normal (or designed this way) that drive is active each second when in Active Idle mode. Well, so that the drive doesn't become lazy and fall asleep on its job, I guess.

Here are some APM values that I have tested with the drive:

    HEX     DEC     RESULT
    ==============================================================
    "01h - 7Fh ... The deepest Power Saving mode is Standby"

    01      1       parked immediately or almost immediately
    7F      127     not tested

    "80h - BFh ... The deepest Power Saving mode is Low power Idle"

    80      128     parked immediately or almost immediately
    96      150     parked immediately or almost immediately
    9B      155     parked immediately or almost immediately

    9C      156     parked in about 3 sec after several read/write sounds
    9D      157     parked in about 1 minute
    9E      158     not tested
    9F      159     parked in about 1 minute
    A0      160     parked in about 1 minute
    A1      161     parked in about 1 minute
    A2      162     parked in about 1 minute
    A3      163     not tested
    A4      164     not tested
    A5      165     parked in about 1 minute

    A6      166     parked immediately or almost immediately
    A7      167     parked immediately or almost immediately
    A9      169     parked immediately or almost immediately
    AA      170     parked immediately or almost immediately
    B4      180     parked immediately or almost immediately
    BE      190     parked immediately or almost immediately
    BF      191     parked immediately or almost immediately

    "C0h - FEh ... The deepest Power Saving mode is Active Idle"

    C0      192     read/write sound approx. each second
    C4      196     read/write sound approx. each second
    C8      200     read/write sound approx. each second
    E0      224     read/write sound approx. each second
    FE      254     read/write sound approx. each second

"The advanced power management levels contain discrete bands, described in the section of Set Feature command in detail."

"When Feature register is 85h (=Disable Advanced Power Management), the deepest Power Saving mode becomes Active Idle."

I provided the quotations from the HGST Travelstar 7K1000 specification in quotes.

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