Under Linux, it is possible to have a /proc filesystem. This directory doesn't contain files, but rather contains information about all the running processes and environment information. Its purpose is to allow programmers to obtain information about the current state of the machine (eg. the ps program). Because it contains run-time information, we obviously don't want to back it up.
taper has an option of trying to detect the /proc system and ignoring it when doing the backup. Normally, the /proc system is on device number 1. If your /proc system is on another device, then you can tell taper this with the -j (--proc-device) option, or preference.
If you are not sure what device your /proc is mounted on, then run the program which_device which is in the utilities menu. Give the name /proc (or whatever name your /proc is) and taper will tell you the device your /proc is mounted on.
Before using taper, it is a good idea to confirm that your /proc is on device 1 and if it isn't change the appropriate preference.