Today I found out, why my USB hard drives behave strangely when connected to my Slackware 14.0 machines.
Seems that on every suspend/resume, pm-utils script /usr/lib/pm-utils/power.d/harddrive executes
hdparm -W 0 -S 0 -B 254 -M 0
on every connected drive.
While this can have unintentional consequences on USB hard drives, disabling the write cache on SATA drives causes a huge performance loss
. It should never been disabled permanently, only for journal flushes, which the kernel does automatically by himself.
I solved that issue by removing the x bit on the /usr/lib/pm-utils/power.d/harddrive, but that gets overwritten by every update.
This is my opinion on this issue: If a machine has no battery and is connected to AC power all the time, such scripts should not fiddle around with any settings (including PCIe power-management and file-system read-ahead, journal commit) and should go with kernel/device defaults instead.