What is the max micro sd capacity on the Guitar board?
Currently bigger SDcards have 32, 64 or 128Gb I guess the limitation he is asking is:
how much the Controller of the Guitar can maximally address ?
Also is a SSD faster then a hard drive?
This is in no relation to the Guitar - a good SSD has a much faster io (input output) usually double as fast than HDD.
If you ask this specifically for the Guitar, your limitation is the USB3 - get the right cable to use it.
USB2, every current 2,5" HDD is more than fast enough.