(from Debian #469692)
7zr does not support multi-volumes, even when they are in 7z format:
$ 7zr t test.7z.001
7-Zip (A) 4.57 Copyright (c) 1999-2007 Igor Pavlov 2007-12-06
p7zip Version 4.57 (locale=en_US.iso8859-1,Utf16=on,HugeFiles=on,2 CPUs)
Processing archive: test.7z.001
Error: Can not open file as archive
$ 7z t test.7z.001
7-Zip 4.57 Copyright (c) 1999-2007 Igor Pavlov 2007-12-06
p7zip Version 4.57 (locale=en_US.iso8859-1,Utf16=on,HugeFiles=on,2 CPUs)
Processing archive: test.7z.001
Testing test.iso
Everything is Ok
(...)
-- System Information:
Architecture: i386 (i686)
Kernel: Linux 2.6.22-3-686 (SMP w/2 CPU cores)
Locale: LANG=C, LC_CTYPE=C (charmap=ISO-8859-1) (ignored: LC_ALL set to en_US.iso8859-1)
Shell: /bin/sh linked to /bin/bash
Probably it's fixed in latest version (9.04).