I have a computer I never use, I was wondering if it's safe to save cryptocurrencies on it using software wallets.
It's safer than storing on a computer that you always use and is connected to the Internet, but that doesn't mean 100% safe. If you computer has ever been connected to the Internet then there is a risk it's compromised. Your best bet is reformatting your drive and doing a fresh install of your operating system. Linux probably works best for that since it's freely available.
Is it also possible to make a copy of the wallet and store it in another harddisk that I'll plug out of the computer?
Yes, but that's not really necessary. A much safer, more convenient way to back up your wallet is use one which gives you a recovery seed (password) which you can write down. See versions of Electrum for that.
Software wallets are safe if they're not connected to the internet, right?
Again, they're safer than ones that are not connected but not 100% safe. Think of who else might have access to that computer. Maybe they don't know money is stored there and start using it without your knowledge. I recommend using a dedicated hardware wallet for any significant amounts. Try Ledger Wallet.
Also: I've got 10 different coins that I want to store, are there any good and secure multi coin wallets out there?
Storing multiple coins securely is much harder than picking a few and just concentrating on storing those correctly or using a hardware wallet. Hope this helps!
