Debian – How to Fix Grub Installation Failure with mdadm RAID1 and GPT

debian Gnu/Linux is supposed to mount HFS+ automatically, and 'just work'--it even has all the packages pre-installed typically. You may need to install hfsprogs and any other "hfsp..." packages you are missing. I've seen disks like yours plug-n-play into Debian.

The support for hfs+ has reportedly (I will post a link, can't find it at the moment) been troublesome, in the most recent kernels, since around Linux 2.7 times, so the workaround is to use the -t hfsplus and -o [all options needed] for mount. Your kernel is probably already telling you to possibly use the force option, and is explaining that this is risky.

There is a really great explanation of how to do this, in this post, and of course more information is on on the man pages for mount and hfsplus.

This greatly assumes that your disk is not messed up, for example containing data from old installations or being otherwise corrupted.

Since you created the disk on a Mac, be sure also the table is actually GPT, and not an apple partition map or even some type of other table or hybrid-table; user sourcejedi recently pointed this out to me.

If you'd like to post an update, I will gladly follow-up on this answer and/or provide more research if you can post the output of viewing the disk in gparted (or gdisk as mgorven suggested above), as well also post information from dmesg from just after all of the above commands and trying to mount the specific partition /dev/sda1 using the correct offset and sizelimit, as I mentioned.

fdisk is the wrong tool for disks >2TB. Use parted or gdisk instead.

It appears that /dev/sdc1 and /dev/sdd1 are 2TB partitions, so that's what limits your array size. For the other disks, they have GPT so I assume they are 3TB already, but you should check.

Basically you have to stop the array, enlarge each partition to 3TB (without changing the starting offset), then start it again and follow it up with a grow:

mdadm --grow /dev/md0 --size=max

If you can't stop the array, you'll have to fail each 2TB partition individually, repartition and re-add it. This might go faster if you add a write-intent bitmap first.

mdadm --grow /dev/md0 --bitmap=internal

Then for each disk individually,

mdadm /dev/md0 --fail /dev/disk1 # check mdstat for [UUUU] first
mdadm /dev/md0 --remove /dev/disk1
parted /dev/disk -- mklabel gpt mkpart primary 1mib -1mib
mdadm /dev/md0 --re-add /dev/disk1
mdadm --wait /dev/md0 # must wait for sync

Once that's done you can remove the bitmap again (keeping it may harm performance).

mdadm --grow /dev/md0 --bitmap=none
mdadm --grow /dev/md0 --size=max

Finally do your resize2fs or whatever.