According to the information on the NickLock homepage and the installation manual, the NickLock is supposed to work with all IDE hard drives. However, I can imagine that some drives could prevent your system from working properly, particularly if they don't have any mode jumper set. Because the NickLock selects drives by closing their master jumper pins, it could be a problem if a hard drive shall not be activated: The master jumper will remain open without the slave jumper being set. Particularly very old drives (< 540 MB) tend to make your system behave strangely if the mode jumpers are not set correctly. That's mainly due to low tolerance in case of jumper configuration mistakes. I tried both a Seagate ST320430A (Barracuda ATA) and IBM's DTLA307030 (DeskStar 75 GXP); both drives worked properly.

Conclusion

The NickLock could be a perfect and inexpensive solution for those who have to share a single computer: Dad can create a computer specifically for the kids by installing the NickLock along with a second hard drive. Leaving the key lock on the second position will ensure that the kids can use the computer without putting his important data at risk. At work, the NickLock could prove a valuable add-on if there is a large number of part-time employees who don't have overlapping work schedules - by sharing a computer, the company could save on hardware costs. Furthermore, when leaving the office at the end of the day, access to the data can be secured by locking the system. Although it definitely isn't meant to be a professional security system, it still provides a convenient way to switch between two hard drives.