Resource busy

Miraculix1

Get feedback from developter, 

 The btrfs warning message is neither new nor common, but the message is new. The error has no effect on data safety or anything related to that.

 In the next firmware release 6.8.1 you should no longer see this message.

There is a topic to discuss the problem, https://community.netgear.com/t5/Using-your-ReadyNAS/6-8-0-Kernel-warning/m-p/1353378#M135290

I didn't find other problem from your NAS,

Could do you please enable SSH and login NAS via SSH?

then run top command to check the resouce loading when you delete files useing finder

By the way,

Did you install MySql? I see your root paratition (md0) is used 81%,  the root paratition is easy to full on your NAS,

You may check ibdata1 size in /var/lib/mysql via SSH login,

below is example,

root@nas-43-97-44:/var/lib/mysql# ls -alh
total 2.3G
drwx------ 1 mysql mysql  182 Aug 16 01:21 .
drwxr-xr-x 1 root  root   298 Aug 16 01:17 ..
-rw-r--r-- 1 root  root     0 Aug 16 01:17 debian-5.5.flag
-rw-rw---- 1 mysql mysql 2.3G Aug 16 01:55 ibdata1
-rw-rw---- 1 mysql mysql 5.0M Aug 16 01:55 ib_logfile0
-rw-rw---- 1 mysql mysql 5.0M Aug 16 01:54 ib_logfile1
drwx------ 1 mysql root  2.1K Aug 16 01:18 mysql
-rw------- 1 root  root     6 Aug 16 01:18 mysql_upgrade_info
drwx------ 1 mysql mysql  868 Aug 16 01:18 performance_schema
drwx------ 1 mysql mysql   32 Aug 16 01:21 sbtest