Hey there, Meddet!
Just like @Petrossa mentioned, the worst thing about it is the fact that you won't be able to get the speed of the SATA 1 HDD (~150 MB/s). If you are pleased from the performance of the HDD so far, the fact that it's connected to SATA 2 (3 Gb/s = 300 MB/s) should affect it anyhow. However, don't expect too much of the gaming performance from such an old drive and make sure you regularly backup the data from it somewhere else as well. This is the surest way to avoid any potential data loss.
Best of luck! Hope we were helpful!

SuperSoph_WD