Epson 20 - Manual (Page 144)

Hard Disk Controller and Hard Disk Check
The computer next checks the hard disk controller and drive unit. If
a malfunction is found in the hard disk controller card, you see this
error number and message:
1782-Disk controller failure
If an error is found in the hard disk drive unit, one of these error
messages appears:
178x-Disk x failure
179x-Disk x error
where x is 0 or 1, and represents the drive number of the hard disk.
Drive C is number 0, and drive D is number 1.
Power-on Diagnostics B-5