Ok, not that many responses so far 
In the meantime I recompiled the kernel for my RPi to be able to use UASP and tested with iozone.
pi@raspberrypi:~ $ zgrep USB_UAS /proc/config.gz
CONFIG_USB_UAS=y
Unfortunately that doesn’t help, RPi can not benefit from UAS:
[ 8.118269] usb-storage 1-1.2:1.0: USB Mass Storage device detected
Here are the iozone results – settings as described here: USB/UAS - linux-sunxi.org
KB reclen write rewrite read reread
4096000 4 34561 34116 36825 36425
4096000 1024 34754 34179 37322 37283
In other words: I get 34.5/37 MB/s with a JSM567 USB-to-SATA bridge using btrfs (choosing any other filesystem or disabling btrfs compression for USB disks attached to Raspberry Pis is just insane 
)
Ok, no need to think about the PiDrive any longer. If the performance is really just ~10MB/s as reported by others and if it’s impossible to query S.M.A.R.T. values and given the long history of WD drives dying way too fast due to the LCC problem when used with Linux using the PiDrive seems to be the worst possible way to access an external HDD.
