Hi Issac Wan,

The main issue is to keep the drive letter for the USB hard drive, right?

We can take the following steps to have a check. Please backup the registry keys before we made modifications to it.
1 Connect your USB HDD and the PC will recognize the driver (For example: Driver E and Driver F)
2 Go to registry, HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
3 You will find some Value named as \DosDevices\E: and \DosDevices\F:
4 Renamed them, (For example, you want to force Driver E to Driver P, rename \DosDevices\E: to \DosDevices\P:)
5 Disconnect your USB HDD and reconnect to have a test.

Best regards

Yes, I've changed back the letter F, but it will change to letter G or H automatically in few days later.

It is best to select a letter between M and Z .If you pick a letter like E, F, or G, the drive letter may still change frequently because these letters may be used for other drives, like CDROM drives, that are not always connected.

I'm using windows 8.1 home edition.

I've plug in the external harddisk and keep it plugged in, never unplug the external harddisk, I've assigned the drive letter F to this external harddisk. Few days later, it will automatically changed to drive letter G or H. I've tried change back to letter F and it reverts to letter G or H in someday. How to solve this problem? Thanks.

Unplug the drive and reboot the computer and check. Plug the drive back in.

Press the WinKey+X to display the system menu and select Disk Management.

Right click the drive you want to change and select the Change drive letter and paths option.

Click the Change button and follow the instructions.

Yes, I've changed back the letter F, but it will change to letter G or H automatically in few days later.