MyBook Mirror WDH2U20000E (2*1 TB) - data transfer speed?

Hello all.

I have recently bought item from subject.

I’m not happy at all, since max transfer speed I have reached is about 22 MB/sec (from another USB HDD).

BUT, both devices are USB 2.0 type.

In theory, max USB 2.0 speed is about 60 MB/s. Practical speed is about 40 MB/s.

So where do I loose 12 MB/s?

It happens under both Win XP Pro SP3 and Win 7 Enterprise 64bit