This applies to the 18i20
There is a known issue between Apple OSX 10.6 Snow Leopard, and the Scarlett 18i20. This incompatability causes this OS version to Kernel Panic/Grey Screen whenever the 18i20 is connected and switched on.
Please note that this particular issue is not known to be consistant through the Scarlett range, and currently only affects the 18i20.
Please also note that the 18i20 is not listed as compatible with OSX 10.6 Snow Leopard, please see the full 18i20 compatibility list HERE.