what back up disk should i use with MAC

See Backup disks you can use with Time Machine - Apple Support. Basically, any external hard disk of your choice.

I and many other experienced members of this community like the OWC Mercury Elite Pro series. Why, extremely reliable, reasonable pricing, fabulous service. I have some that are over 15 years old that still work fine. All HD's die however with these they are not throw away enclosures, when the HD dies you can install a new HD in under 5 minutes with a simple screw driver. Something that cannot be done with many of the others.

I use the same drive enclosures that rkaufmann87 recommends and wholeheartedly agree with his assessment.

Other reasons/benefits:

1) The Mercury Elite Pro uses standard 3.5-inch hard drives. Those are much more reliable and robust, and usually faster, than the little 2.5-inch laptop drives that are used in cheap "mobile" or "portable" external drives. I've never had a 3.5-inch drive fail. I've had three 2.5-inch drives fail. They are simply not made for the long haul.

2) The OWC drives have an independent power supply. The small portable drives can only get power from the computer's USB ports. Sometime the drive demands more power than the USB port can provide. The majority of external drive complaints here—largely about connectivity problems—are about the little portable models.

The OWCs are more expensive than the cheap drives that seem designed to go on sale every other weekend rather that to give long reliable service. But what do you answer when you ask yourself, "How much is my data worth?"

I buy the bare enclosures from OWC and install the very good WD "Black" series drives rated as 7200 rpm with transfers at SATA 6GB rates. Plenty fast, and installing a drive in one takes one screwdriver and maybe five minutes if you stop to pet the cat.

IHMO, an SSD is overkill for a Time Machine drive.

You’re welcome.