Seagate IDE Momentus spins down and immediately up again

My new 2.5" Seagate IDE ST92011A Momentus drive regularly spins down and
immediately spins up again, this can happen multiple times per minute.
My Windows Power Management is configured to spin down the disk after 15
minutes.

Running Seagate's Seatool diagnostics software shows no errors. Anyone
know what could be causing this?

The drive is in an old ATA33 laptop, I use Seagate's Dynamic Drive
Overlay to circumvent the bios HD size addressing limitation, the OS is
W98. The system bios's own power management is disabled, the system is
ACPI & APM capable, ACPI is enabled in the bios.

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Spartanicus