To andretim: your suggestion to call directly FileCore_SectorOp is unfortunately a short term solution. The architectural best solution is extend FileSwitch (the mother of all FS, including FileCore) for >31bit file size access, even directory enumeration and file info and catch other fish by this as well (like >2GByte files on FS which support this (like most network based onces)).
The fact that FileCore needs to be used when ImageFS needs direct access to its media is not logical when we are talking about media which are not supported by FileCore. Eg. CDFS. Currently you can't neatly support other CD/DVD fileformats just by implementing a ImageFS. Typically this is worked around by writing a FileCore based FS talking to CDFS which then make FileCore say "oops, this is not a FileCore formated media; let's ask around for a suitable ImageFS".
It is wrong that Service_IdentifyDisc is FileCore specific, it should be possible to let CDFS/USB Mass storage issue a Service_IdentifyDisc alike service call directly (without the need to have a FileCore based FS on top of CDFS/USB Mass storage) so that if you don't have FileCore in RISC OS, this still would work.