I bought a MR-1000 when they first came out (over 10 years ago) and successfully replaced the internal with I think a 120GB, I will check when I get home from work. It was the largest size replacement at. the time. Works fine. I now have 3 MR-1000s, and was going to upgrade the two others to bigger hard drives. The stock hard drive is 40GB, and although that may sound large... and average show can be about 14-15GB. So doing a festival, you are going to max out after three bands. So having bigger hard drive just made it easier. There should be an old post here somewhere where I showed how I replaced the hard drive. It's an older 1st gen type ZIFF that has a ribbon cable interface. I do know that someone tried using an adapter to try using a newer SSD and never got it to work. Only the same type ZIFF drive can be replaced. I tried calling Korg direct to ask them details, and there were ZERO help - BUT I did confirm that the OS for the Korg is built into a ROM chip, even if you do Firmware updates. Why is this important? At first we thought the OS was on the internal drive, so we were "cloning" the original 40GB to a newer larger one (so that we had the Korg OS "on" the hard drive). You would install it, then reformat the hard drive to see the larger size hard drive. The cloned drive would appear as 40Gb unless you did a "format", then the hard drive would jump up in size. I can confirm you just put in a new SSD and the deck will still boot up and you can just format the new drive. No need to clone the OS onto the drive you want to install. This took a month to figure out... back in 2012.
So how did you confirm that 100GB was the largest size? Did you try installing a big hard drive. I want to install 256GB drives. What replacement drives are you using to upgrade, I want to do my other 2 Korgs. I don't think there is a size limitation, but the Korg is very finicky as to which ones work. They need to be the 1st gen older models.
Also, the 12V AC in is a non-standard pin. I am in the process of replacing that connection with a standard 12V locking pin, which will then allow me to run a SmallRig V Mount Battery VB99 via D-Tap out to (important: 12V regulator built inside cable) to 12V locking 5.5mm, same connector as Lunatec V2 <-- which I use with the Korg. I can run the SmallRig battery via D-Tap out and power both the V2 and the Korg. The AC power supply that comes from Korg is NOT a standard 5.5mm plug.
I run ALL my gear (MixPre 10ii) via D-Tap batteries now. I got special cables made by Alvin's Cables on Amazon to regulate the 18V coming out of D-Tap to 12V max! So as not to fry the Lunatec V2 or the Korg. I can post more details about the cables, but they are easy to purchase right on Amazon.
I will post photos of the cables I am using later tonight.