Do I need to use cloud hosting for clients’ data?

He is looking to meet quota for the month and charge you for something that you don't need.

You do NOT have to use cloud hosting. 

How in the world would some stranger over the phone know whether your computer hard drive is secure or not? 

ProSeries hosting is quite expensive, hes looking to make a sale.

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@Jiaai wrote:

otherwise client's data can be stolen, proseries is not safe.

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No computer is completely safe.  Not even a cloud hosting company.

However, there are many things that can be done to increase safety.  A cloud hosting company has IT-security experts that regularly monitor their computers, implement processes and install programs and/or hardware to increase safety. 

Do you have a IT security expert/company that checks and maintains your computer system and works with you to take precautions to increase safety?

Things like good passwords, Multi-Factor Authentication, anti-virus and firewalls are the first line of defense.  A good IT-security expert will set you up with that, but they also can do much more to decrease the likelihood of problems.