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Why would private keys be lost from a wallet.dat file that doesn't seem to have been corrupted. Nothing hints so far to possible file corruption issues or did I miss something in gigi0ne's posts?
If the magnetic recording on the HDD's platter surfaces degraded enough to cause data corruption, the user would get read-errors and problably would've had severe issues to get a working wallet.dat from such a situation.
HDD technology and bit density of devices before and around 2009 weren't particularly problematic to assume you can't read the data after about 15 of non-use. (OK, we don't know anything about the drive's storage conditions.) It's not flash memory technology where trapped charges in flash cells can dissipate over time, especially more problematic with multi-bit cells.
Harddrives that aren't used for many years most commonly have other problems, mostly spin-up related and/or lubrication of motor spindle bearings.