Just to clarify one point of confusion in this thread: (quotes to follow)
Here's the Apple KB about it.
The same article does seem to imply that System versions prior to 7.5 placed a 2GB volume limit on local hard disk partitions. If someone has a whopping big drive in a machine that can run 7.1 or earlier it might be an exercise for the reader to see if it will allow a partition larger then 2 GB.
The Wikipedia article on HFS concurs with the 2 Terabyte absolute limit. The real problem of course is since it's limited to 16 bit's worth of allocation blocks the minimum file size is 1/65,536th of the whole disk. This is similar the limitations on FAT16 DOS partitions, which is why Microsoft *did* set a maximum partition size of 2GB for them. (Just to be pedantic, Windows NT upped that to 4GB but consumer versions of Windows never supported the larger size properly.)
So, in short, a 6100 *should* support a giant hard drive with an IDE converter formatted to full capacity under System 7.5, but you'll be wasting tonnes of space. You'd be much better off running OS 8 or even 9 on it. (assuming you have the RAM for the latter)
The 2GB figure the Apple KB article is referring to is in reference to the largest volumes that can be reliably used by elderly versions of the Appleshare file sharing protocol. Mounting network volumes larger then that will cause unpredictable behavior. This is unrelated to local hard disk size limits.
Here's the Apple KB about it.
The same article does seem to imply that System versions prior to 7.5 placed a 2GB volume limit on local hard disk partitions. If someone has a whopping big drive in a machine that can run 7.1 or earlier it might be an exercise for the reader to see if it will allow a partition larger then 2 GB.
The Wikipedia article on HFS concurs with the 2 Terabyte absolute limit. The real problem of course is since it's limited to 16 bit's worth of allocation blocks the minimum file size is 1/65,536th of the whole disk. This is similar the limitations on FAT16 DOS partitions, which is why Microsoft *did* set a maximum partition size of 2GB for them. (Just to be pedantic, Windows NT upped that to 4GB but consumer versions of Windows never supported the larger size properly.)
So, in short, a 6100 *should* support a giant hard drive with an IDE converter formatted to full capacity under System 7.5, but you'll be wasting tonnes of space. You'd be much better off running OS 8 or even 9 on it. (assuming you have the RAM for the latter)