Sandisk SSD USB not detected by Android
There are one major differences between an USB stick and an USB SSD: power consumption

As SSDs usually don't have the possibility to use a separate power supply you have to place a active USB hub (a hub with an own power supply) between the phone and the SSD.

The SSD is optimized for high performance and therefore requires much more power than a simple USB stick. 

The Sandisk Extreme Portable SSD 500 GB requires about 4 Watt.
Usually smartphone only support USB 2.0 and therefore the output is limited to 5V at 500 mA = 2.5 Watt. Only a very few smartphone support USB 3.0 or USB power delivery in OTG mode which would make it possible to use an SSD via OTG.

On a regular computer the SSD works because USB 3.0 allows 900 mA (5V at 900mA -> 4.5 Watt) and therefore it can power the SSD correctly.