Partition, Format and Mount new disk in Debian

Find the disk

lsblk

Note the name of the device, e.g. /dev/sdb

Create a partition

Welcome to fdisk (util-linux 2.39.3).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help):

1. Create a new label by pressing g
2. Enter n for a new partition.
3. Choose the size you want
4. Save changes by pressing w

Format the partition

Mount the disk persistently

Create the folder where the new disk is going to be mounted:

Edit /etc/fstab

# <file system>    <mount point>    <type>    <options>    <dump>    <pass>
/dev/sdb1          /mount-path      ext4      defaults     0         0

Test the mount:

If no errors, run

Resize the partition

This should become its own article, but here we go:

Unmount the partition:

Then follow the Guide on Stack Exchange

Enlarge the partition:

p to print the partition table, take note of the number, start, end, type of sda1.

Delete it: d

Recreate it using command n with same number (1), start and type but with a bigger end (taking care not to overlap with other partitions). Try to align things on a megabyte boundary that is for end, make it a multiple of 2048 minus 1. Change the type if needed with t (for partitions holding an extX or btrfs filesystem, the default of 83 is fine).

Then w to write and q to quit.

The partition table will have been modified but the kernel will not be able to take that into account as some partitions are mounted.

However, if in-use partitions were only enlarged, you should be able to force the kernel to take the new layout with:

If that fails, you’ll need to reboot. The system should boot just fine.

Then, resize the filesystem so it spreads to the extent of the enlarged partition (might be located in /sbin):

Which for ext4 will work just fine even on a live FS.