MAGFAST's magnets are about 4mm by 4mm by 10mm and they are made from neodymium. This is the most powerful type of magnet: powerful enough to snap two MAGFAST devices together and hold them firmly in place, even though very small.
The strength of magnets is measured as 'pull force' or 'pull strength' - the force required to pull a magnet away from a steel plate.
Assuming that MAGFAST specifies the strongest possible neodymium magnets, N52, we calculate the pull force of a magnet at around 2.6 lbs. Strong, but nowhere near as powerful as many of the other magnets already in your home, in speakers, your refrigerator door, your desktop computer and printer. Your cellphone contains tiny neodymium magnets in the speaker, camera, receiver, haptic feedback device, and more. There are also larger magnets in headphones, the speakers in your laptop and tablet, and hard drives themselves contain much larger neodymium magnets - strong enough to pull about 10 lbs.
Magnets and Storage Media
If you're interested to know more about what it would take for a powerful magnet to erase data on your hard drive, see this article by the team at KJ Magnets. Even a huge neodymium magnet with a pull force of over 600 lbs had no effect on the data of a hard drive with the power turned off.
You can be confident that the tiny magnets used in the MAGFAST range are not powerful enough to have any effect on a hard drive or similar modern storage media.
It is nevertheless true that you should keep even relatively small magnets (including those in your phone) away from older types of magnetic media such as the stripe on your credit card or hotel room key card, or audio and video tapes.
Magnets and Medical Devices
Research indicates that magnets can interfere with the proper functioning of sensitive medical devices such as pacemakers and implantable cardioverter defibrillators.
Once again, it is unlikely that the tiny magnets in MAGFAST devices are sufficiently powerful to cause meaningful interference to modern medical equipment. However, while MAGFAST’s products are thoroughly tested to meet all modern standards, we can’t validate the shielding and design of all medical devices and you should contact your doctor or the provider of your medical device before using any electronic equipment (including as cellphones which could cause electromagnetic interference) close to any sensitive medical equipment. This heart.org article may help you identify some of the products that could interfere with a cardiac device.
MAGFAST does not warranty its products for use in conjunction with or in close proximity to any medical device.