"An external flash for digital Nikon" means that it is a TTL or iTTL flash.
Each TTL auto flash does not work well because it can not communicate with the camera. This usually means that it is triggered at full power.
A simple manual flash works, but you'll need to do a lot of calculations using the flash output index and subject distance to get the correct aperture for the right exposure.
The most suitable flash is an older film age flash with an auto thyristor sensor on the front of the flash, which automatically adjusts the flash output to the aperture and ISO selected on the camera.
Some older film era Nikon flash units also used car thyristor. Newer Nikons such as the SB-800 and the SB-900 also have an auto-thyristor option.
A good example of an older car thyristor flash is the Vivitar 283.
Instruction manual Vivitar 283