What causes a computer to recognize some ext. drives with USB but not others. The old comp recognizes drives that the new machine won't.?

some usb devices are old and need More power(in fact some need a usb splitter to provide power from TWO ports, and ports are in pairs so that may mean one from each of two pairs whilst the other one of each pair has nothing connected). most laptops these days have several usb ports, some are high power some are low power. maybe you need to be a bit more careful what devices are plugged into what ports. Thus an external device might NOT START cos it aint getting enough amps.

The only thing I can think of is that the drive isn't getting enough power from the USB port to work. This can be an issue with those smaller laptop USB drives. The larger desktop USB drives usually have an external power cord because they need more power than the USB cable can provide.

In this case, the new computer's USB ports may not be powered, or may not be powered like the older computer's ports are. Despite the "U" in USB standing for "Universal" you'd be surprised how much variance there is. :-(

So if your hard drive can use an external power adapter, try that, and just use the USB connection for data transfer - not power.

The drives are compatible with Windows 7, but not compatible with Windows 10.

Drive capacity can be a factor. Old operating systems couldn't address drives with a capacity over X. Sorry, I've forgotten what the limit was.