Replaced failing disk in NV+ v2 - both volume expansion and volume scan fail

I found some solution

https://community.netgear.com/t5/ReadyNAS-Beta-Release/4-2-16-T75-problem-s-ultra-6-w-six-3-TB-drives/m-p/799924#M470

but

root@nas-JohnyB:~# apt-get install lsof
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
libupsclient1 libpcap0.8 cracklib-runtime libnss-mdns libcrack2 libcap-ng0
libavahi-client3 libnet1
Use 'apt-get autoremove' to remove them.
The following NEW packages will be installed:
lsof
0 upgraded, 1 newly installed, 0 to remove and 107 not upgraded.
Need to get 282 kB of archives.
After this operation, 455 kB of additional disk space will be used.
Err http://ftp.us.debian.org/debian/ squeeze/main lsof armel 4.81.dfsg.1-1
404 Not Found [IP: 128.61.240.89 80]
Failed to fetch http://ftp.us.debian.org/debian/pool/main/l/lsof/lsof_4.81.dfsg.1-1_armel.deb 404 Not Found [IP: 128.61.240.89 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

I don't now how umount /c :/ for offline resizing