I'm planning to connect some old SSD what I've already to my RasPi3B. I ordered cheap SATA-USB adapter cable from ebay. It has two USB-a plugs, one is for data and power another is only for power.
Can I plug this power plug to USB charger and other to RasPi?
I quess it will also try power RasPi's USB connectors via adapter, is it bad?
Adapter I ordered
The only way you will know is to try it.
Frankly, I wouldn't do this - you are effectively connecting 2 different switch mode power supplies together.
You don't say how much current the SSD needs, so it is not possible to give a definitive answer.
If the Pi has an adequate power supply it SHOULD work with just a single USB connection (the Pi has no power control for individual USB ports - you can draw 1.2A from a single port).
If there are problems (and the Pi USB does have problems with high current devices) you would be better using a powered hub.
This is possible with two power supplies. You need to disconnect the "+5v" from the usb connector between the PI and the SSD. That way you only have common GND, and that will allow communication with no double 5V supplies. Admitted not the most easy thing to do.
