Welcome on board, PLASTIK!
EOS does not boot from a hard drive. The operating system is stored in the CPU Flash RAM.
Because you have "just" 1 MB CPU Flash RAM, you can only update to EOS 4.10 (classic).
You can also upgrade the Sample RAM up to 128MB.
Other expansions like "2nd MIDI port" or "AES/EBU digital I/O including an ASCII keyboard interface" are not available any more. They were standard on E4XT and E-Synth Classic. Maybe you can find them if someone sells these parts because his unit diet.
The AES/EBU expansion comes with a AES/EBU receiver chip that has to be installed on the mainboard.
The ADAT I/O expansion can be found from time to time on ebay and required EOS 4.
Regarding the hard drive: EOS supports up to 18GB storage media.
Keep in mind that hard drives can die the older they get. You should think about a backup solution or an alternative storage media.
Some people use a Compact Flash cardreader that is connected via SCSI to the sampler. As this cardreaders all have IDE interfaces, they are connected to a SCSI-IDE bridge adapter to be connectable to the SCSI interface. Search the forum for more information.
You can build such a device by yourself or get a turn-key solution at
http://www.scsiforsamplers.com. This cardreader also works for AKAI if you still have yours.
Another issue a lot of users reported, including me, is that the rotary encoder is not working properly any more because of worn out contacts. Search the forum for more information if this issue occurs to you, too.
Cheers,
Robert