Is it capable to have a higher drive capacity? I know it isn't really possible to replace the 20MB HDD in a real unit, but is there anything in the HD20 protocol to limit you to 20MB? (or 32MB? or 2GB?..) :-D
I'm too lazy to find the link to back this up (it's in one of the old threads about the HD-20), but the communication protocol for the HD-20 includes a method for the drive to communicate its size to the host. The HD-20 protocol was designed to allow any sort of "drive-like storage device" to be attached; the hard drive icon that appears on the screen when an HD-20 is running is actually stored in the HD-20's ROM, and it uses linear block addressing, IE, sectors are numbered from zero to (the number of sectors on the drive) similarly to SCSI.
So, yes, in theory if someone were to make an HD-20 emulator it should be able to accommodate disks larger than 20MB without having to change the ROM code/HD-20 init. (Within reason, of course. There's undoubtedly some hard limit on the number of sectors the drive ID data structure can communicate, probably somewhere south of 2GB.)