After turning time machine on and then off again for an external volume, the disk will spontaneously unmount and takes around 30 seconds to list directories. Disk utility can't find anything wrong with it. Does anyone have any idea what could be the issue?
how old is the drive?
Also try smartmontools http://sourceforge.net/apps/trac/smartmontools/wiki
sounds to me like a drive soon to go bad. =/