Brand new external hard drive is not detected, and when it is, it disconnects.

You MAY have a power problem, Any USB2 port can supply to a connected device up to 0.5A current. There are almost NO external HDD units that can run on that power limit. Usually the symptoms, when you try this, is that the drive looks like it is trying to work, but fails every time you actually try to access for reading or writing. A few rare machines have USB2 ports that can supply more power, but that is not the USB2 standard.

To deal with this reality, the original USB2 "laptop drives" had two options. Some came with a special two-header USB cable, and you had to plug BOTH of the two plugs on one end into different USB2 ports on the host computer. Others came with their own "wall wart" power supply module that plugs into the wall and into the drive, and you MUST use this power supply.

The new USB3 system raised the current limit to 0.9 A. It is entirely possible to design a "portable laptop external drive" that includes a low-power HDD unit inside that CAN work within that limit. So now, almost all "Laptop Hard Drives" are specified to be used with USB3 systems and come with a new USB3 cable to connect to the host computer's port. Many of these do NOT come with any extra power supply module or special two-headed cable because that is not necessary IF you use it with a real USB3 port. But they still can NOT work when plugged into a USB2 port with a lower current available. This can be confusing, because many of them are described as being "USB2 compatible". And so they are in the sense that the DATA TRANSFER systems between the two USB systems are compatible and the connectors will fit together. The just do not make it clear that it will not work that way unless you provide a power solution.

So, OP, what type of USB port are you trying to plug this drive into? AND, what type of USB cable are you trying to use for that? Both the host computer's USB port and the connecting cable ought to be of the USB3 variety. If you don't have the right port and cable, post back here. I know a way to arrange to power a USB3 external drive while connecting it to a USB2 port on a host computer, although the data transfer rate will be the slower USB2 rate. If you need that, post back here and I'll provide details. Also, post the exact model number of the Seagate external drive unit you have, in case a power supply module is available for it.