Summary: Don't open a CK savings account.
Never seen any savings offering that let you link only one account and doesn't let you change it for that matter. Second, you never see the account number nor the routing number associated with it, so you cannot pull money away. Last 4 digits aren't exactly helpful.
Several people, including me, got into a bind where the linked account has been closed, so you effectively lose a way to withdraw or deposit into the CK account. What's the point of the offering if you can't do anything with it anymore? Right, none. CLOSE THE ACCOUNT.
They warn you several times that if you close the account then they won't let you open a new one. With the external account restriction combined with information hiding, I'm fine with that.
Closing an account with them takes place through their support which is over email! Which means it can take several days to do it. The slowest.
I don't know what weird shit CK is offering here, but it isn't a savings bank account.
Yeah, use another provider that is proper and actually lets you use it like the bank account that it is.