I removed the harddisk (only one screw at the bottom necessary) and attached it to my Ubuntu machine with an external harddisk adaptor.

• fdisk -l shows which partition the data is:
  Medium /dev/sdc: 931,5 GiB, 1000204886016 Bytes, 1953525168 Sektoren
Einheiten: sectors von 1 * 512 = 512 Bytes
Sektorengröße (logisch/physisch): 512 Bytes / 512 Bytes
I/O Größe (minimal/optimal): 512 Bytes / 512 Bytes
Typ der Medienbezeichnung: dos
Medienkennung: 0xbb3b9c17

Gerät Boot Start Ende Sektoren Größe Id Typ
/dev/sdc1 63 1028159 1028097 502M 8 AIX
/dev/sdc2 1028160 1953520064 1952491905 931G 20 unbekannt
  /dev/sdc2 in this case.
• I needed to install the administration tools for the RAID tools: apt-get install mdam
• mdadm --examine /dev/sdc2 shows that the partition is just a normal software RAID with one device:
  
Magic : a92b4efc
Version : 0.90.00
UUID : 46c2be7d:ee8709a0:5f6ad052:539aa5e2
Creation Time : Fri Dec 30 00:34:45 2005
Raid Level : linear
Raid Devices : 1
Total Devices : 1
Preferred Minor : 0

Update Time : Fri Dec 30 00:34:45 2005
State : active
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0
Checksum : 18e472f9 - correct
Events : 1

Rounding : 64K

Number Major Minor RaidDevice State
this 0 8 2 0 active sync /dev/sda2

0 0 8 2 0 active sync /dev/sda2
• Look which /dev/md* is still free. In my case, I could use md1. mdadm -A -R /dev/md1 /dev/sdc2 creates a block device from the RAID partition.
• Now the block device /dev/md1 can be mounted and unmounted as usual.
• The block device should be removed again with mdadm -S /dev/md1