The /drive0 and /drive1 folders corrspond to the /dev/hda4 and /dev/hdc4 devices.
Those are partitions on the two drives. These partitions gets mounted into the single (Linux doesn't use drive (partition) letters) filesystem at boot time. Everything is joined under the single root of the filesystem ( / )
The /driveX folders are the best place to stash stuff, as they are large (the system partitions are IIRC 16MB) and also unaffected by software upgrades.
The reason you can't upload stuff is that the partitons per default are mounted read-only. Through the ftp client you can issue a "site rw" command to remount stuff read/write. Don't forget to issue a "site ro" before rebooting - otherwise you'll be in for a fsck (rather lengthy procedure) on then sync.
Standard
FAQ pointer. However, the ftp stuff will (is not yet) be covered in the Hijack part of the FAQ.
/Michael