How to unlock an ATA password-locked HDD with an upper-case password if the BIOS accepts only lower-case?

This is a "feature" of many BIOS implementations. Fortunately, the ATA security spec is only enforced by the BIOS, not the drive itself. That means if you can get around the BIOS, you can access the drive.

Please note, a USB enclosure may not work. Issuing ATA commands to a SATA drive in a USB enclosure may fail. It depends on whether or not the USB controller in the enclosure will reject the commands.

Here's a better way:

1. Burn yourself a Linux Live CD or make a bootable Linux USB flash drive. There are many tools and ISOs available on the Internet for this.
2. With the computer turned off, unhook the SATA cable from the affected hard drive.
3. With the SATA cable disconnected, power the computer up and boot from the CD or flash drive you made in step 1.
4. Once the system has booted, reconnect the SATA cable to the hard drive. This is called hot-plugging.
5. IF your laptop supports SATA hot-plugging (some don't), then your hard drive will become available and you can use hdparm to remove the password from the drive.

If your laptop does not support SATA hotplugging, then this trick will have to be performed on another computer -- preferably a desktop, since desktops are usually more standards-compliant than laptops are.

Also, the SATA controller must be set to AHCI for this to work. IDE or RAID modes will fail.

I succeeded by plugging the drive in an old PC and issuing hdparm --security-unlock PWD /dev/sdb. The drive is now reusable.