Has anybody seen this issue? I have solved it for myself but still am baffled as to why this happened.

I used "mke2fs -cj /dev/hdbXX" to create some partitions, then did "fdisk -l" and the new partitions showed up as FAT. They worked, and I was able to write to them while installing a new distro, but they would not boot up from GRUB. It would give an error "FS type unknown, partition type 0x6".

Here's the backstory: I was mucking around with my partitions so I could install some new distros to try out, and decided to do it via windows XP's "manage computer" option since I didn't trust my old version of Partition Magic on such a big hard drive (250GB). I had a partition that was like 30GB reserved for another linux install, so I deleted it and split it up into 4 separate unformatted partitions.

So then I booted into my Fedora Core 3 distro, and went about formatting the partitions (using mke2fs) as ext3 so I could install FC5, Suse 10.1, etc. Of course at first I thought everything was fine and started installing distros and realized they wouldn't boot (see above). Then I checked fdisk and it still thought they were FAT partitions.

I eventually solved it by first using parted to delete and recreate the partitions, then formatting them using parted. (I didn't like how parted reordered the partitions, since the partitions were in the middle of the disk but it gave them the highest disk numbers (hdb13, 14, 15, 16) - so I recreated them in windows.) The trick is to NOT use mke2fs to format, but to use parted to format the partitions if the partitions were created using windows. The question I have is... why? Anyone have an idea?

IIRC ext3 should just be -j not -cj, but i could be wrong.

Well, partition type set by parted or fdisk is property written in partition table. Partition contents - FS, for example - is written entirely on partition itself. Maybe you could simply set partition type in fdisk - it shouldn't even touch data on partition, but I wouldn't bet on it.

I have always used cfdisk to make my partitions. It seems to be easier to use, and I have never had a problem with it. fdisk always gives me problems. so try cfdisk next time.

Well, it's not a matter of a program, it's matter of forgetting to change partition type.

So what else would I have had to do besides run "mke2fs" to actually change the partition type correctly? I take it mke2fs only does the actual formatting and does not change the type?

Yes. Try running cfdisk and change type of the partition (big button type) anw write this. I don't think there is very big risk of losing data on partition, but nevertheless backup.

I have lost data before playing around with the partition table. Make sure you back up before you do anything,