Started happening after I got to Morgan Everett's place.
I do have a ridiculously large number of saves, but I've deleted a whole bunch of them and I still can't make any new saves, past my first save at Everett's, without it overwriting the most recent one.
Basically: first save at Everett's is fine, second save was overwritten by the third, which was overwritten by the fourth, and so on.
Looks like it's happened to other people before - http://www.gamefaqs.com/boards/250533-deus-ex/60370099 - but I haven't had any luck googling a fix for this.
The neuro-paralytic gas causes "no inconvenience whatsoever", does it, Toby Atanwe? You gave me anterograde amnesia, you bastard!! --- John was fully aware he could do this all alone. Thomas...hoped he'd never have to.
Checked the save files within the DeusEx folder. Once you hit 1,000 save folders, you can't make any more until you delete them from the top!
The thing that was messing me up was that the latest save is always recorded as the highest number. Doesn't matter if you deleted save folders 1-999, if you still have folder 1,000, you can't make any new saves, even if it's the only save you have!
Changing the names of the folders borks your saves, so the only solution is to delete a whole wad of saves from 1,000 downwards, until you get to a low enough number. Thankfully, the Quick Save will act as a bookmark for you, so you won't lose your progress, even if you lose your saves. --- John was fully aware he could do this all alone. Thomas...hoped he'd never have to.