Q: why is the iDE mod only available on the SCPH 700xx version of the PS2 Slim?
A: For the moment, ide mod is only available on SCPH700xx, as this is the only revision where Sony has kept the IDE management circuit on the motherboard, as on FAT. They then withdrew it from subsequent revisions for cost reasons I think, as these components were of no use on the Slim versions, and to my knowledge there has been no official hard disk on the PS2 Slims.
Q: What is the maximum usable storage with iDE mods?
For the IDE to SATA mod (e.g. Super Playstation 2), there’s no real limit to the use of OPL with exFAT support (tested up to 4TB without any problem).
For traditional OPL versions, the limit is 2TB, which can be interesting if you want a direct boot with a chip on your HDD/SSD. In that case you’ll need to go through the HDL Batch installer to add your games to your drive, whereas with the OPL exFAT version you just copy/paste your ISOs to your hard disk.
For the mod with the SD card reader, the popular adapters were historically limited to 128GB because of the memory chip used alongside the FC1307 controller. This limit is now removed: with our Custom Firmware pre-installed adapters, you can use SD cards well beyond 128GB — up to 1TB and beyond.