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I used bcache to cache my 24GB SSD using this tutorial. I tried to back up my HDD with:

sudo make-bcache -B /dev/sdb1

But I get this error:

Can't open dev /dev/sdb1: Device or resource busy

My file system looks like this:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb1       423G   79G  323G  20% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            3.9G  4.0K  3.9G   1% /dev
tmpfs           788M  1.4M  787M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            3.9G  916K  3.9G   1% /run/shm
none            100M   12K  100M   1% /run/user

lsblk looks like this:

sda      8:0    0  22.4G  0 disk 
└─sda1   8:1    0  22.4G  0 part 
sdb      8:16   0 931.5G  0 disk 
├─sdb1   8:17   0 429.5G  0 part /
├─sdb2   8:18   0     1K  0 part 
└─sdb5   8:21   0   7.7G  0 part [SWAP]
sr0     11:0    1  1024M  0 rom 

Is there anything wrong with my setup that might explain the error?

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That tutorial appears to be for setting up a new system and the partitionsare unmounted and (I assume) empty.

In your case you appear to want to set your backing device as your currently mounted root filesystem. This is already deeply at odds with the tutorial. It's just not going to work like that.

But from its comments, somebody points out g2p which appears to claim to be able to convert existing sytems. I haven't used it so I honestly can't speak for it but seriously, make sure you've backed up before you do anything else. There does seem to be a very real risk here that you'll have to reinstall at some point.

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