see here
if u create ur btrfs with 2 hard disks with raid1 metadata and raid1 data that is , examble below

mkfs.btrfs -L Test -m raid1 -d raid1 /dev/sda /dev/sdb

in this case of two hard disks
all files will be stored 2 times ( one copy of each file on each hard disk ) and if u remove one hard disk

u can mount (the attached hard disk to ur PC) with
mount -o degraded /dev/sda /mnt/Test

u can recover ur data here
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now you wont be able to recover ur data if

u created mkfs.btrfs and added 3 hard disk at a time with a raid1 data and raid1 metadata such as 


mkfs.btrfs -L Test2 -m raid1 -d raid1 /dev/sda /dev/sdb /dev/sdc

in this setting , data is divide and spread on all hard disk such as a 1gb file will be stored as

333 mb on sda
333 mb on sdb
3333 mb on sdc

sure u can remove 1 hard disk and mount the remaining 2 in degraded but that 300 mb in hard dish which is not connected is not going to be there so

just stick with 2 hard disk , do scrub command daily , and all good

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the geparted shows true disk usage and
"btrfs filesystem show" command shows somewhat fake disk usage so

check disk usage in geparted .