Hi,
I just did a fresh install booting from a custom cd and copying the 98se files on the HDD. Now it's up and running, I spent hours trying to install a driver for my usb key before I realised that if i put a "lastdrive=h" command in the config.sys, the drive actually does show. My config.sys was empty at first. Why does it limit the amount of drives to the original config (3 partitions +2 optical drives) ? Where, in what file, is the "lastdrive=g" hidden ?

When DOS boots, it allocates a drive letter to each partition. When the Win32 system loads, it extends this to cover the optical drives. It's not hidden anywhere. If you had 18 partitions, you would end up with drives to the letter T.

If you mean to set lastdrive, you should do this in config.sys.

Nope. Maybe I'm not clear. By the way, the problem is solved, this is a question, not a cry for help.
See, if I add a partition, letters are shifted and my last optical drive disappears because the drive letters is limitted to my original line up: 3+2. This is not normal IMAO. I can solve it by extending the "lastdrive" to H or I, but this is not normal IMHO. There must be something wrong somewhere.