Two parts to this.
a. you storing data on the Safe Network - this data is stored on other nodes after being split up into chunks. It prob never see any of it on your disk drive.
b. Other people who are storing data will have their data stored on various nodes across the network. At some stage some of that data from everyone else will see some chinks stored on your computer.
As has been mentioned, wallets only store the keys to the address the coins exist on and the actual coins/tokens are data stored on the respective network (Be it the BTC network or the Safe network). SafeCoin is renamed to SafeNetwork Token to reflect its actual existence as a token to buy resources.
Safe Network does not do mining. To earn the token (SNT) you are rewarded for successfully retrieving chunks from the store of chunks the network asked you to store. This is unlike mining which is doing crypto maths (usually on a GPU) to validate transactions etc.
No wallet exists that supports SNT (nee SafeCoin) in the wild since SNT (nee SafeCoin) has never existed yet, except as a test token on a testing network.
There are 2 at least coins out there with SafeCoin as their “name”, one is a from a group with some sort of crypto project and others faking either it or the SafeNetwork in name only. I am wondering if you came here from reading about one of the other “SafeCoin” projects which do use crypto mining.
Safe Network is primarily a data storage network using a token to buy resources and be rewarded for retrieving data. It uses crypto algorithms to keep the token safe on the network, but is not a crypto project since it does not do the “mining” or “blockchain” ledger that a crypto project does.
There will be certain things to consider here and may change that some minimums and maximums will be set by the Network itself as its running. But at this time you set the amount of storage you want to provide for your node.
An external SSD will work fine. Suggest that it has a USB 3.0 interface to get the advantage of the speed of a SSD. Successful serving of chunks will require your node responds reasonably fast so a slow external drive/interface may reduce the amount of rewards you would otherwise get. (Note the use of “may” as its not a definite)