My LC III has a 120MB hard drive. Thing is with System 7.5.3 installed, MS Word, Clarisworks and 2 or 3 games and its almost full.
What is the maximum hard drive i can put in it without modification?
Is there a certain type of hard drive or will any SCSI hard drive work?
I think you can put any size SCSI disk in there and it will work. Mine has a 520MB internal and a 18GB external. Just remember that a non-Apple drive will need a patched copy of Drive Setup or a third-party formatter like Hard Disk Toolkit.
I am running 7.5 with my PPC7100 occasionally. I have two drives attached to that Mac´s SCSI bus:
IBM DDRS-34560, 4,5GB
IBM DNES-309170W, 9,1GB with a 80? to 50 pin adapter.
Both drives are running with a single HFS partition without errors for years.
Newer SCSI devices should be backward compatible to (Mac) SCSI-1 with a suitable adapter, I know there are SCA to 50 pin adapters, but I did not try.
Hope you´ll find something at a reasonable price!
I seem to remember 4GB as a limit at the time, not tested as physical drives topped out at 3.,2GB at the time. It sounds like this wasn't an issue at all.
It was probably an OS-imposed limit. I think System 7.1 only saw 4GB of my 18GB disk when it was first initialized. I have it partitioned up into 2GB chunks anyway to keep the cluster size down.
Isn't it a 32-bit limit? I think that 32-bit registers can address no more than 4GB. The 680x0 chips are 32-bit processors and System 7 is a 32-bit OS.
I found a 1GB hard drive in a box at home its a scsi 50 pin drive. I put it in the mac and switched on but it will not boot, not even from cd.
I did the shift, cmd, delete, option thing but all i get is the floppy disk with a question mark.
When i remove it and put back the miserable 120mb one it can boot from cd and hd no problem.
Whats wrong here?
Check the SCSI ID of the 1GB drive and make sure it's not the same as the one for the CD.
Does the 1GB spin up as it should?
Do you have something like the SCSI "Power Sniffer" ?
If the drive sets the bus to "busy" and doesn´t release it, the Mac will wait forever to boot from CD.
Any chance to connect the 1GB with the original drive in place?
I checked the SCSI ID. The external CD-ROM is 3. I tried the HD in 0, 1 and i think 6 and it didnt work.
The HD spins when switched on. There is a jumper to make a 15 second delay but thats disabled.
The HD was not taken out of a Mac. Protocol 7, you mentioned something about the Apple ROM... Maybe thats why it wont boot not even to CD?
I tried with no HD connected and it wont load CD, with the 1GB HD and it wont load CD.
Will only load CD with the crappy (but working) 120mb original drive connected.
Probably bad or no termination of the SCSI bus at the 1GB drive.
The bus MUST be terminated on both sides to work.
Sometimes it works without terminators on one side, most often not.
What brand is the drive? Can you find a data sheet about the terminators?
Do you dare to open the case of the CD-ROM and cram in the HDD, so you could use the Box´s termination?
If it's not the ID then it could be termination like 24bit mentioned. I can't see how a non-Mac formatted harddisk would prevent you from booting from CD. Is there a jumper on the drive for termination? Maybe if you look up the model number you can see.
The make and model is Connor CFP-1060S which i had pulled from a broken external SCSI case (which i threw away)
Here is its manual / date sheet
http://stason.org/TULARC/pc/hard-drives-hdd/conner/CFP-1060S-1062MB-3-5-...
I don't really understand anything about termination of scsi as i only ever really worked with IDE drives so any help will be appreciated 
I'm not overly familiar with it either but it looks like you need to put a jumper on E8 to enable the internal termination. Basicaly if something SCSI doesn't work, the first things to check are ID and termination.
Could you find these on your drive? If they have been pulled, the Mac wont start with it as internal HDD.
"Setting the Bus Termination
---------------------------
This drive provides on-board Alternative 2 active termination for the
SCSI bus. The termination resistors, which are contained in two
Single Inline Packs (SIPs) should be removed from the drive unless it
is a SCSI device at the physical end of the bus."
E8 already has a jumper 
My guess is a termination problem or a bad drive.
Your Apple drive should have those SIPs on it. The usually looked like little yellow/brown combs on the drive´s logic board near the SCSI bus connector. Maybe you can exchange them?
SCSI-1 could hold 7 devices on a 6 yard daisy chain, terminated by resistor packs on each end.
Terminators can be on the ext. device´s case (at its free connector) or on the SCSI device itself.
Some cases had little switches too, to turn termination on or off.
What is at your CD-ROM device, how is it terminated?
