how to password protect an external disk drive

From > Encrypt and protect a storage device with a password in Disk Utility on Mac - Apple Support

"When you format an internal or external storage device, you can encrypt and protect it with a password. If you encrypt an internal device, you must enter a password to access the device and its information. If you encrypt an external device, you must enter the password when you connect the device to your computer."

Eject and disconnect the external drive.

Then reconnect the external drive to your computer.

Does it not ask for the password when reconnected to your computer?

It is never rude to ask a question.

Because the Encryption option is not available to you via Finder, that suggest it you need to use Disk Utility to Encrypt the external drive. Just beware that you will need to copy everything to another drive and then back, because using the following directions will erase the drive.

see > Encrypt and protect a storage device with a password in Disk Utility on Mac - Apple Support

Hi,

You can protect your external drives with encrypt password.

How to encrypt external drives using Finder

1. Connect the drive you want to encrypt to your Mac.
2. Open Finder and right-click the drive from the Finder sidebar.
3. Select Encrypt.
4. Enter a strong password of your choice, plus a password hint.
5. Click Encrypt Disk.

No, drives have both a Map Scheme and an OS Format.

Open Disk Utility and select that external drive in the list of External drives.

Selecting the external drive will show the Partition Map Scheme.

Selecting the drives indented volume will show the Format.

According to Disk Utility the Hard Drive is a Hitachi, which was used in a lot of PCs and even some older Mac's. Plus it has two volumes, one is named System and one is named Gateway.

Please tell us, how is the old PC drive is connected to your Mac.

No problem, you're welcome.

Personally I would not rely on that old of a drive for anything important.

I would copy everything or anything that was important, to a newer external drive.

If you feel the need to encrypt the newer drive, you should do that first and then copy to it.

You need to select and reformat the entire Hitachi drive, not just the System or Gateway volumes.

Again, beware because Reformatting and Encrypting will erase everything on the drive.

In terminal what does this report?

diskutil list

You said that drive is from a PC and I suspect that the format is preventing you from Encrypting it.

What does "Get Info" report about that drive?