Open Start → Administrative Tools → Computer Management, and select Disk Management on the left side of the computer management window. At this point, a disk device with the same capacity as the mobile hard disk will appear on the right side of the window. Right-click the newly discovered "active" device icon, select "Change drive letter and path", then click "Add" in the pop-up window, select the drive letter (if H is selected) in the combo drop-down menu on the right side of the window and confirm, and the system will automatically add the mobile storage device.

If it doesn't work, it should be caused by insufficient power output of USB interface of your computer or high power demand of motherboard of mobile hard disk. Voltage instability will cause many inexplicable problems, similar to what you said.

Because if it is a large-capacity mobile hard disk, the demand for power supply is relatively large. At present, the power supply of mobile hard disk is mostly done through USB interface, and this large-capacity mobile hard disk will generally test the USB power supply ability of computer. If the mobile hard disk has two USB ports, you should try to plug both input ports on the USB cable of the mobile hard disk into the computer, which should meet the power supply requirements. But if it doesn't work, you need to buy an external power supply, which will usually solve the problem.

Another thing to note is: check whether there are jumpers on your current hard disk (you need to disassemble the machine, if you don't understand, go to the local computer shop, and it will be broken if it is worse). Because some hard disks don't have jumpers, which means they are slave disks by default, you should set the jumpers on the hard disks as master disks and try again. Setting them as slave disks conflicts with mobile hard disks.