I need to transfer or copy hard disk to cds, but new computer does not have a slot for hard disk. Is there a convert available to copy from disk to cd?

If your new computer does not have a cd/dvd drive, you will need to look into external cd/dvd drives. There are several models available. The ones I have seen connect to your computer by a usb port.

Is there a reason to copy to cd's. Would a flash drive work instead?

A CD can hold a max of about 800 g of data, Even a DVD is limited to a little over 4 G. Most New PCs have hard drives of 500G or more.

So maybe if you explain what you need we can advise further.