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Drives are powered by the USB bus, they are in SATA-to-USB enclosures (one is AgeStar and other is Transcend). I also tried to connect them (not both at the same time, separately) using a powered USB hub - no success.
When the disk is connected, I hear the sound of initialization, it spins up, but then system becomes unstable, freezes, the disk tries to mount and than goes offline. It repeats until I unplug it.
"blkid" shows only SSD partitions, and "fdisk -l" too.
For more information, any of the disks works great with ASUS HDP R1 multimedia player, which has exactly the same power unit as Efika Smarttop.
I know it's weird, and I have no idea what to do further...
That they're powered by the USB bus - root hub or externally - is probably exactly why it fails. Both the Smartbook and Smarttop USB ports are the external ports of a powered USB hub which provide the same power so there is no difference really.
In the end, a rotating metal platter takes more power than a single USB port should be able to provide. It is a fluke of the boost converter on the Asus board that it works, not the DC power supply powering it, so the fact they are identical means nothing..
You are at the mercy of the converters on the board and the ones in your powered USB hub. It both needs to somehow create a 6-8W spike on disk spinup *and* do it on a 12V bus which is not what USB provides.
Your Asus system is probably designed with a freakishly flexible boost converter powering the USB bus. I've got an Asus motherboard myself (pretty old actually) that can power 10W from a single USB port, and a Via Nano which will supply so much power on the USB bus that it actually burned my Beagleboard before it blew out the power supply. It seems some PC vendors rely on devices not drawing past the maximum current, and the overvoltage/overcurrent protections aren't implemented. This might be bad for your disk.
Go buy a power supply for your disk, if you can. This isn't so much an Efika problem as a problem of trying to use magnetic media from USB bus power and happens all over the place.