How to mount a USB drive partition as read/write?

Question

I have a USB drive with three partitions:

• sdb1 is an EFI partition
• sdb2 is an HFS+ partition
• sdb3 is an MSDOS fat partition with Kali linux on it

I can boot into Kali linux using this USB stick on a MacBook and the sdb2 partition shows up on the desktop as HD (the name I gave it when I formatted it on the Mac).

As referenced in this thread, I installed hfsprogs and tried:

sudo mount -o force /dev/sdb2 /media/root/HD

The partition is still read only.

sudo df -hT

gives

Filesystem     Type      Size  Used Avail Use% Mounted on
udev           devtmpfs  7.8G     0  7.8G   0% /dev
tmpfs          tmpfs     1.6G  9.5M  1.6G   1% /run
/dev/sdb3      vfat       26G  3.4G   23G  14% /lib/live/mount/findiso
/dev/loop0     iso9660   2.8G  2.8G     0 100% /lib/live/mount/medium
/dev/loop1     squashfs  2.5G  2.5G     0 100% /lib/live/mount/rootfs/filesystem.squashfs
tmpfs          tmpfs     7.8G     0  7.8G   0% /lib/live/mount/overlay
overlay        overlay   7.8G  356M  7.5G   5% /
tmpfs          tmpfs     7.8G     0  7.8G   0% /dev/shm
tmpfs          tmpfs     5.0M     0  5.0M   0% /run/lock
tmpfs          tmpfs     7.8G     0  7.8G   0% /sys/fs/cgroup
tmpfs          tmpfs     7.8G     0  7.8G   0% /tmp
tmpfs          tmpfs     1.6G   24K  1.6G   1% /run/user/0
/dev/sdb2      hfsplus   3.7G   25M  3.7G   1% /media/root/HD

I tried to unmount the partition:

sudo umount /media/root/HD

and get

umount: /media/root/HD: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)

So, it seems that Linux thinks the HD partition is busy for some reason. Maybe because the boot partition is sdb3? Here is some more info that may help.

issuing:

sudo fdisk /dev/sdb

gives

Welcome to fdisk (util-linux 2.28.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

A hybrid GPT was detected. You have to sync the hybrid MBR manually (expert command 'M').

I have no idea what that means. So then I tried:

fdisk -l

which gives

Disk /dev/sda: 465.9 GiB, 500277790720 bytes, 977105060 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: D4AAF120-967C-41B9-9FA4-EB4EBD806D19

Device         Start       End   Sectors   Size Type
/dev/sda1         40    409639    409600   200M EFI System
/dev/sda2     409640 898785143 898375504 428.4G Apple HFS/HFS+
/dev/sda3  898785144 900054679   1269536 619.9M Apple boot
/dev/sda4  900055040 977104895  77049856  36.8G Microsoft basic data


Disk /dev/sdb: 29.6 GiB, 31742492672 bytes, 61997056 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: E93D7B7D-38FD-404B-9163-63AAA9A39ED5

Device       Start      End  Sectors  Size Type
/dev/sdb1       40   409639   409600  200M EFI System
/dev/sdb2   409640  8010815  7601176  3.6G Apple HFS/HFS+
/dev/sdb3  8272960 61734871 53461912 25.5G Microsoft basic data


Disk /dev/loop0: 2.9 GiB, 3076767744 bytes, 6009312 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x77e6cfe3

Device       Boot   Start     End Sectors  Size Id Type
/dev/loop0p1 *         64 5794271 5794208  2.8G 17 Hidden HPFS/NTFS
/dev/loop0p2      5794272 6009311  215040  105M  1 FAT12


Disk /dev/loop1: 2.5 GiB, 2634285056 bytes, 5145088 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

You can see the Kali Linux partition on sdb3 which I had formatted as MSDOS fat. How can I get sdb2 to be read/write? Or even unmount it?