If your copy of Manhunt running on the PlayStation 2 hangs at the save screen, the solution is to simply use an 8MB memory card. Even though the error message that eventually shows up says that an official Sony memory card is needed, the issue specifically has to do with the size of the memory card for some reason. According to the current reverse engineered documentation of the PS2 hardware: for unknown reasons, certain games do not work if the memory card is larger than expected. If you are using Open PS2 Loader (OPL) virtual memory cards WILL work, they just need to be set at 8MB. Due to the bug mentioned above you should only be using virtual memory cards of 8MB for all games.
I was not going to do one of those recipe website moves where I provide a whole background story before giving the answer people are looking for. If you're confused as to why this is posted here, I felt that this post fell in with a lot of the one off technical solutions I have been posting as of late. I ran into this issue about a year ago, meant to share a broader explanation to the solution online, and am only now getting around to it. So if this post saves one person hours of trouble because they're able to find this on Google, great!
(And if you're asking why post something like this in the first place:) Manhunt was a niche stealth PS2 game developed by Rockstar Games in 2003. The stealth mechanics developed in the game have been implemented in just about every Rockstar game released after. Even in their most recent release, Red Dead Redemption 2, the stealth mechanics and games play and feel exactly like Manhunt. Due to the violence and subject matter of the game, there was an ensuing media circus and the game was outright banned in some countries. Users are unable to play the game on other platforms today because the re-releases and ports are considered inadequate and buggy compared to the original release. As a result many, users run into the above error early on when trying to run the game on the original hardware. For the sake of media preservation and to humor those with an interest in the forbidden or obscure - I think this is important information to share.